bluuebunny/crossref_metadata_2025 huggingface dataset:
A Ferroelectric Thin-Film Transistor Based on Sputtered HfZrO and IGZO
Authors: Ryu Suhwan , Seong Tae-Yeon
Published: December 2020

In recent years, because of increased demand in IoTs devices, wearable displays, circuits, sensors, and other high technology devices, the thin film transistors (TFTs) gated by ferroelectric film as memory devices are of considerable interest. Ferroelectric (FE) materials including Pb(Zr0.4Ti0.6)O3 (PZT), (1-x)[Pb(Mg1/3Nb2/3)O3]-x[PbTiO3] (PMN-PT), and polymer poly (vinylidenefluoride-trifluoroethylene) (P[VDF-TrFE]) have been used to realize ferroelectric transistors with nonvolatile memory. Particularly, HfO2-based materials have attracted a great deal of attention since the breakthrough in the ferroelectricity of Si-doped HfO2, which can overcome some of limitations of current ferroelectrics such as scalability, lead-free materials and suitable for high-temperature process. Moreover, HfO2-based materials exhibit better performance in terms of a switching characteristic than low-k gate insulator (SiO2) as well as high-k materials.

In this study, the performance of sputter-deposited ferroelectric hafnium zirconium oxide (HfZrO)/high-k HfO2 gate dielectric for a-IGZO TFTs on glass substrates was investigated. The sputterd HfZrO film was crystallized with orthorhombic phases after rapid-thermal-annealing at temperature of 400 °C for 30 min in air and exhibited ferroelectricity. The performance of the FE-TFT based on HfZrO/HfO₂ (15 nm/185 nm) was compared to typical TFTs fabricated with PECVD-SiO₂ (200 nm) and sputtered HfO₂ only (200 nm) gate insulators. The transfer characteristics of the FE-TFTs were demonstrated with low operating voltage of 0.1 V, one order higher on-off ratio and drain current than the typical TFTs with the SiO₂ and HfO₂ gates, high field effect mobility 5.04 cm2V-1s-1 and small subthreshold slope (SS) about 117 mV/dec. The low operating voltage and small SS results of the HfZrO/HfO₂-based device indicates that the use of the HfZrO/HfO₂ could serve as a potential gate for the fabrication of high-performance FE-TFTs.
P3HT:ZnS Based Photovoltaic Devices with Enhanced Performance Assisted by Oxidised Carbon Nanotubes
Authors: Wei Chaohui , T. Bishop Matthew , Wang Yifei , Gao Fei , Wang Chenxu , Chen George Zheng
Published: December 2020

Photovoltaic devices based of P3HT:ZnS bulk heterojunctions currently produce relatively low efficiencies, limiting the scope these devices have for further development. This work aims to produce a novel photoactive layer, by incorporating oxidised carbon nanotubes (CNTs) into a P3HT:ZnS bulk heterojunction, to investigate the potential CNTs in improving these devices. Furthermore, the active layers of these photovoltaic devices were created using a single-source precursor, thereby limiting potential barriers for scaling up this process to industrial scale. It was found that the CNTs that had been only mildly oxidised (LowOx-CNTs) were longer in length compared to the higher oxidised CNTs (HighOx-CNTs). This allows the HighOx-CNTs to disperse more evenly throughout the active layer, due to the increased oxygen functional groups on the surface of the CNTs, although this does reduce their comparative conductivity. Due to the reduced conductivity of the HighOx-CNTs, 5 and 10 wt% of CNTs performed at lower levels than that of the LowOx-CNTs, with the most notable difference at 5 wt% (0.0064 % and 0.0096 %, respectively). However, LowOx-CNT devices resulted in lower fill factors (FFs), likely due to increased recombination, which can be linked to the increase path length from the increased size of LowOx-CNTs. As the load of the CNTs increased, the FFs for both devices decreased and the Jsc values began to plateau at roughly 0.15 mA/cm2. As the Jsc plateaus, the beneficial conductive properties of LowOx-CNTs was overcome by the detrimental recombination mechanisms introduced by the longer path length. This caused the HighOx-CNTs to outperform the LowOx-CNTs after 15 wt% doping with a power conversion efficiency of 0.0122 %. This increases the efficiency of the undoped ZnS device by more than 7 times, which can be further increased with the incorporation of electron and hole transporting layers.
Phototransistor with Heterogeneous Double Layer Composed of CsPbIxBr3-x Perovskite and in-Ga-Zn-O Semiconductor for Visible Light Detection
Authors: Na Hyun-Jae , Cho Nam-Kwang , Lee Sung-Eun , Lee Eun Goo , Kim Kyungho , Lee Jaehak , Kim Youn Sang
Published: December 2020

All inorganic perovskites with good excellent physical properties received a considerable attention in optoelectronic applications. Despite the high-performance photoelectric properties of CsPbI3 perovskites retaining the α-phase, actual applications of the perovskite film were hindered by the phase instability at ambient conditions. To improve the availability and long-term stability of CsPbI3, multi-anion structures have been studied in the ABX3 structure of CsPbI3. Herein, we suggest inorganic heterojunction with CsPbIxBr3-x and In-Ga-Zn-O (IGZO) semiconductor which applied bi-anion IxBr3-x instead of I3 in CsPbI3. Using this heterojunction, we demonstrate a new phototransistor with a structure of p++ Si / SiO2 / IGZO / CsPbIxBr3-x / Ti-Al-Ti where IGZO is a charge transfort layer and CsPbIxBr3-x is a light absorption layer. The material stable properties of CsPbIxBr3-x were confirmed by changing the ratio of I : Br from 10: 0 to 0:10. When the ratio of CsPbIxBr3-x perovskite I: Br is 8: 2, the inorganic perovskite maintains the black α-phase in the ambient condition. Phototransistor based on CsPbIxBr3-x perovskite/IGZO heterojunction shows excellent responsivities both ultraviolet region and entire visible light region, unlike the single-IGZO phototransistor reacting only in the ultraviolet region. the phototransistor exhibited a resistivity of 26 A/W, a detection of 8.42 x 1014 Jones, and an external quantum efficiency of 51% at the visible light region (620 nm). Specially it showed the excellent stability over 1 month at atmosphere (20 °C, 25 RH%).
Hybrid Quantum Dots/WS2 Nanowalls Bifunctional Devices for Efficient Optical and Gas Sensing Applications
Authors: Tang Shin-Yi , Yang Chun-Chuan , Su Teng-Yu , Yang Tzu-Yi , Wu Shu-Chi , Hsu Yu-Chieh , Chen Yu-Ze , Chueh Yu-Lun
Published: December 2020

Transition metal dichalcogenides (TMDCs) have not only recently attracted tremendous attention due to their unique optical and electrical properties but also the interesting and various nanostructures created by different synthesis process. These atomically thin TMDCs materials possess great potential in sensing, optoelectronic, energy harvesting and Li-ion battery applications. However, the atomic thickness of TMDCs will limit the light absorption and result in weak performance of optoelectronic devices, such as photodetectors. Here, we demonstrate a novel approach to increase the surface area of TMDCs in the one-step synthesis process of TMDC nanowalls from WOx into WS2 nanowalls. By utilizing the rapid heating and rapid cooling process, we can achieve the formation of nanowalls with the height of ~150 nm standing perpendicularly on top of the substrate. Our method provides a rapid synthesis process with enhanced aspect ratio compared to the conventional solid-vapor phase CVD process, which is commonly used for the synthesis of planar TMDCs with few atomic layers. Moreover, the combination of colloidal quantum dots (QDs) with three different emission wavelength and WS2 nanowalls will further improve the performance of WS2-based photodetector devices, including 3.5~4 times photocurrent enhancement and shorter response time. The remarkable results of the QD-WS2 hybrid devices to the high NRET efficiency between QDs and our nanostructured material are caused by the spectral overlap between the emission of QDs and the absorption of WS2. Additionally, the outstanding NO2 gas-sensing properties of QDs/WS2 devices were demonstrated with a remarkably low detection limit down to 50 ppb with a fast response time of 26.8 s contributed by tremendous local p-n junctions generated from p-type WS2 nanowalls and n-type CdSe-ZnS QDs. Our work successfully reveals the energy transfer phenomenon in QD-WS2 hybrid devices and shows great potential in commercial multifunctional sensing applications.
Synthesis of High Entropy Oxide through Sol-Gel Method
Authors: Wu Ping Yun , Wu Jyh-Ming
Published: December 2020

High entropy oxides composed of Ag, Al, Zn, Cr, and Cu are synthesized through sol-gel method. The metal nitrates, including silver nitrate, aluminum nitrate, zinc nitrate, chromium nitrate, and copper nitrate, are firstly dissolved in distilled water and stirred for one hour at room temperature. After mixing thoroughly, citric acid will be added into the solution as the coordinating agent to catch all metal atoms and stirred for another half an hour. Then, by adding ammonia, the pH value of the solution is adjusted to seven and the solution is dried in the oven at 90oC for 48 hours. After that, the powder was annealed at 300oC for one hour to obtain the final product. The x-ray diffraction pattern shows that AgAlZnCrCuO exhibited a single-phase spinel structure with the second phase of CuO. Interestingly, based on the scanning electron microscope image, the AgAlZnCrCuO nanoparticles show highly porous morphologies, suggesting that the AgAlZnCrCuO may possess the unique potential application for electrochemical process.
Unusual Thermoelectric Anisotropy in Rhombohedral Germanium Telluride
Authors: Askarpour Vahid , Maassen Jesse
Published: December 2020

Developing high performance thermoelectrics to convert waste heat into useful electrical power is an important challenge that has the potential to impact our energy future. This requires exploring and discovering new materials with enhanced electronic and thermal (phonon) transport properties. Advances in computational modeling now allow the thermoelectric characteristics of materials to be calculated fully predictively, and thus represents a powerful tool to be partnered with experimental efforts.

In this talk, I will present our recent findings on rhombohedral GeTe, a layered quasi-2D material predicted to have unusual anisotropic transport properties [1]. Our approach combines density functional theory with the Boltzmann transport equation, to extract the detailed electron and phonon dispersions along with the electron-phonon scattering rates, in order to predict the thermoelectric transport coefficients. Our results show that the electrons preferentially conduct along the cross-plane direction, while the holes favor the in-plane direction. Interestingly, this feature enhances the thermoelectric figure-of-merit ZT for n-type GeTe, since the conductivity (and power factor) is maximized perpendicular to the atomic layers while the lattice (phonon) thermal conductivity is minimized - leading to a three-fold increase in ZT. The cross-plane power factor and mobility in n-GeTe reach roughly 32 μW/cm-K2 and 500 cm2/V-s, respectively. An analysis of the band structure reveals that the large cross-plane transport originates from high-velocity conduction states, formed by the Ge p-orbitals that span across the interstitial region. These findings illustrate how the dominant electron and phonon transport directions are effectively decoupled in n-GeTe; a feature that can potentially be found in other materials to achieve enhancements in thermoelectric performance.

[1] V. Askarpour and J. Maassen, “Unusual thermoelectric transport anisotropy in quasi-two-dimensional rhombohedral GeTe”, Phys. Rev. B 100, 075201 (2019).

Figure 1
Fabrication of Two Component Passive Frequency Filters By Using CdTe Nanostructured Thin Films Electrodeposited from Ionic Liquid Medium
Authors: Chauhan Khushbu , Mukhopadhyay Indrajit , Patel Dipal B
Published: December 2020

The passive frequency Filters are mainly made up of ceramics or piezoelectric materials that are mainly used in receivers, mixers and radio transmitters. Very limited studies have been done to obtain filters by a simple configuration of R-C elements which are the basis of these devices. In this study, 2 component passive filters were realized by using the Schottky junction between FTO and CdTe nanostructured thin film. CdTe nanostructured thin films were directly deposited on FTO substrate via electrodeposition from ionic liquid medium. A simple configuration of Cu : FTO : CdTe : Cu was utilized to attenuate the frequencies from 7 kHz to 0.7 MHz. Interestingly we have found that by changing the dimensions of CdTe nanostructures, the cut off frequency as well as band width of such filters can be varied. This study is first of its kind which shows the direct relationship between dimension of CdTe nanostructures and their frequency response
(Invited) Smart Textiles Towards Sustainable and Pervasive Energy Future
Authors: Chen Jun
Published: December 2020

Energy crises and global warming severely limit the ability of human civilization to develop along a sustainable pathway. Increasing renewable energy sources and decreasing energy consumption are fundamental steps to achieve sustainability. By using remotely deployed sensors, the Internet of Things (IoT) has already changed our daily life in fundamental and meaningful ways. In the meanwhile, it is also forcing the world to use a new form of energy to sustainably power billions of distributed devices/sensors. The required energy source needs to be pervasive, mobile and beyond the capability of the traditional centralized, ordered power supplying systems. Batteries may not be the best solution for the IoT, owing to their limited lifetime, size, and environmental concerns. Powering the IoT would be impossible without making the sensors self-powered by harvesting energy from the working environment to ensure the long-term operation. In this talk, I will introduce our research that contributed to the sustainable energy future via three fundamental approaches (1) energy harvesting from the ambient environment, especially I will present a hybrid power textile, which was constructed via large-scale weaving technique for simultaneously harvesting energy from ambient solar radiance and human body motion. (2) energy saving via developing a variety of self-powered/low-power sensors. (3) energy saving via promoting the energy efficiency. Especially, I developed a nanophotonic structure textile with tailored mid-infrared property for passive radiative human body cooling to improve the building energy efficiency. The nanoporous polyethylene fabric could lower human skin temperature by 2.3 oC compared with the commercial cotton fabric of same thickness, contributing to a greater than 20% saving on indoor cooling energy.
All-Inorganic Perovskite Solar Cells Using Treated Carbon Materials for Effective Charge Extraction
Authors: Zhou Cheng , Chen Xiang , Zhang Meili , Pang Beili , Dong Hongzhou , Sun Qiong , Feng Jianguang , Yu Liyan , Dong Lifeng
Published: December 2020

Perovskite solar cells (PSCs) have become the leader in the field of the third-generation solar cells due to their outstanding photovoltaic performance and low cost. However, the instability stemming from low tolerance to humidity and heat as well as residual solvents and ultraviolet illumination would be challenging for long-term stability. Recently, all inorganic perovskites, especially carbon-based hole-transporting material (HTM) CsPbBr3 perovskite, have attracted increasing attentions because of their enhanced moisture and thermal stability compared to hybrid counterparts.

For the CsPbBr3 device, a challenging route to increase power conversion efficiency (PCE) is to take interfacial charge transfer into consideration and reduce interfacial energy-level differences to reduce space charge accumulation and enhance charge extraction. It is known that graphene-based derivatives are promising candidates for interfacial materials due to their excellent electronic properties. For example, graphene quantum dots (GQDs) between perovskite and TiO2 could significantly enhance the performance of PSCs due to dramatically enhanced electron extraction by the inserted GQDs; polyaniline/graphite composites were incorporated into carbon electrode to tailor work function and to improve hole-selectivity of back electrode for enhanced energy level alignment and interfacial hole extraction, leading to a remarkably reduced energy loss and charge recombination.

Herein, we developed a strategy to use carbon dots (CDs) with tunable bandgaps and high absorption coefficients for electron extraction performance. The CDs were prepared by exfoliation method from oxidized graphite, and CDs in dimethylformamide solution were spin-coated onto FTO/c-TiO2/m-TiO2 substrate, then a multistep solution-processed method was used to prepare Cs-Pb-Br films with controllable CsPbBr3 phase. Specifically, CsBr/methanol solution (~15 mg mL-1) was spin-coated onto PbBr2 film and then annealed at 250 °C for 5 min, and the processes were repeated for 5 times. The obtained cesium lead bromide films were washed by 2-propanol. Finally, a carbon back-electrode with an average area of 0.09 cm2 was deposited onto the device. We fabricated all-inorganic PSCs with the FTO/c-TiO2/m-TiO2/CDs/CsPbBr3/carbon structure. Arising from the suppressed interfacial charge recombination, the optimal device yielded a PCE up to 10.67% under one sun illumination. These findings suggest that graphene-based derivatives can be used as interfacial modification materials toward high-performance carbon-based HTM-free PSCs.
Synthesis of Metal-Organic Framework-Derived Carbon Supported CoFe2O4 with Enhanced Oxygen Reduction Reaction Catalytic Properties
Authors: Li Zongzhao , Sui Jing , Zhang Qian , Yu Liyan , Dong Lifeng
Published: December 2020

Metal-organic frameworks (MOFs) have been demonstrated to be promising precursors or templates to construct high-performance electrocatalysts. Among them, Fe2Co-MOF exhibits excellent catalytic performance in oxygen reduction reactions (ORRs). We developed a simple hydrothermal reaction to prepare Fe2Co-MOF, and after heating at 600 ℃ for 2 h, we obtained a new composite catalyst CoFe2O4@carbon nanorods (CoFe2O4@CNR). The composite structure can effectively alleviate the agglomeration of metal particles, make transition metal nanoparticles uniformly dispersed on graphitized carbon nanosheets under the action of non-covalent bonds, ensure structural stability and protect the whole electrode electrochemical activity. Compared with traditional spinel structure compound CoFe2O4, the CoFe2O4@CNR nanocomposite shows excellent ORR performances with an onset potential of -0.2 V vs. Ag/AgCl, a half-wave potential of - 0.34 V and a diffusion-limited current density of 5.05 mA.cm-2, which are better than CoFe2O4 with obvious superior difference of 100 mV, 90 mV and 3.07 mA.cm-2
，respectively. The findings above demonstrate that a simple and efficient method was developed for the synthesis of MOF-derived carbon-supported spinel oxides for ORR electrocatalysts.
Ceria Nanodots Anchored on ZIF67-Derived Framework By Atomic Layer Deposition for Efficient Bifunctional Oxygen Electrocatalysis
Authors: Liu Ziqi , Andrade Angela Macedo , Grewal Simranjit
Published: December 2020

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are the two most important reactions for electrocatalytic activities, has been widely studied for sustainable and renewable electrochemical applications, including fuel cells[1]
,
[2], metal-air batteries[3] and water electrolysis[4]. For the development of new catalysts alternative to noble metal-based materials, metal organic frameworks (MOFs) are being actively used as the precursor structure. ZIF-67, a kind of MOF structure made of cobalt nitrate and 2-methylimidazole ligands, has proved to be a good candidate for ORR catalysis due to their rich of Co-N-C active sites. However, ZIF-67 tends to aggregate during the pyrolysis process by breaking C-H or C-O bonds.

In this study, atomic layer deposition (ALD) was applied to coat an atomically thin cerium oxide (CeO2, ceria) coatings the MOF structure, which is expected to incur synergistic catalytic activity and suppress the thermal agglomeration. ALD is a self-limiting surface chemistry process, and is able to create highly conformally layer-by-layer thin film on any complex 3D structure with controllable thickness at an atomic level.[5] Ceria has shown good synergistic effect with other metals for OER performance due to its ability for electron exchange and ion reduction.[6] We demonstrates a facile synthesis of Co/Ce co-doped porous carbon nanomaterials by pyrolyzing the ceria-coated ZIF structure. The resultant samples showed uniform crystal structure which are in well accordance with ZIF-67 precursors. The as-prepared samples also showed enhanced electrocatalytic activities for both ORR and OER comparable to that of noble metal benchmarks (Pt/C and IrO2) and in terms of onset potential, half-wave potential, and current density. A sample prepared by performing 15 cycles of ceria ALD followed a pyrolysis at 700 °C shows the highest onset potential (0.95 V versus RHE) for ORR and superior stability in comparison to commercialized Pt/C. Meanwhile, a sample with 5 cycles of ceria ALD exhibits an excellent OER performance (1.48 V at 10 mA cm-2) and maintained a good OER durability performance (from 1.48 V to 1.50 V after 2,000 potentiodynamic cycles).

References

[1] W. Xia, R. Zou, L. An, D. Xia, S. Guo, 2015, 8, 379.

[2] X. Wang, H. Zhang, H. Lin, S. Gupta, C. Wang, Z. Tao, H. Fu, T. Wang, J. Zheng, G. Wu, X. Li, Nano Energy
2016, 25, 110.

[3] M. Khalid, A. M. B. Honorato, H. Varela, L. Dai, Nano Energy
2018, 45, 127.

[4] B. Y. Xia, Y. Yan, N. Li, H. Bin Wu, X. W. (David) Lou, X. Wang, Nat. Energy
2016, 1, 15006.

[5] C. Guerra-Nuñez, Y. Zhang, M. Li, V. Chawla, R. Erni, J. Michler, H. G. Park, I. Utke, 2015, 7.

[6] S. Xu, C. Lv, T. He, Z. Huang, C. Zhang, J. Mater. Chem. A
2019, 7, 7526.
(Invited) Understanding Phonons and Thermal Transport in Metal-Organic Frameworks
Authors: Wieser Sandro , Kamencek Tomas , Bedoya-Martinez Natalia , Schmid Rochus , Dürholt Johannes Peter , Zojer Egbert
Published: December 2020

The thermal conductivity is an important property of metal-organic frameworks (MOFs). For gas storage applications, a too low thermal conductivity might turn out to be a game-stopper. Conversely, for thermoelectric devices the thermal conductivity has to be low enough. The chemical and structural flexibility of MOFs provides handles for conveniently tuning thermal properties. Still, for realizing the full potential of these materials, a detailed understanding of the corresponding structure-to-property relationships is required. For achieving such an understanding, we employ quantum-mechanical and force-field based simulations to study heat transport processes in MOFs both in real as well as in reciprocal space. The former approach relies on analyzing the effective local temperature in different parts of the MOF during a non-equilibrium molecular dynamics simulation. This allows identifying specific elements and bonds within the MOF that act as heat-transport bottlenecks. For a variety of MOF structures we show that the main local inhibitor to heat flow is the bond between the metal ions in the nodes and the carboxylic oxygens in the linkers. Further computer experiments then allow assessing the impact of the strength of that bond and of parameters like the mass of the nodes, the length and density of the linkers, bonding motives, the network topology, etc.. Typically, we find that stronger bonds, lighter nodes, and longer and more dense linkers increase thermal transport. As an alternative approach, analyzing heat transport in reciprocal space enables us to connect the values of the thermal conductivity to the properties of phonons as the quanta of mechanical and thermal lattice energy. Of particular interest in this context is the impact of structural variations on quantities like phonon densities of states, group velocities, and phonon lifetimes, which all directly impact heat transport processes. An analysis of these quantities provides insight into the nature of the vibrations that most strongly impact thermal conductivities. Consistent with the results from the real-space analysis, we find that the group velocities of the relevant phonons are increased by lighter nodes and by an increased mode-linker coupling. Notably, the bonds between nodes and linkers are also a major source of anharmonicities, thus reducing phonon lifetimes and decreasing thermal conductivities. Finally, we show that phonon properties are also crucial for other parameters, like elastic constants, thermal expansion, and - by virtue of electron-phonon coupling - also charge transport.
(Invited) High Performance Stochastic Memristive Networks for Neurocomputing and Neurooptimization
Authors: Strukov Dmitri B.
Published: December 2020

Stochastic neural networks have become the state-of-the-art approach for solving problems in machine learning, information theory, and statistics. The key operation in such networks is a stochastic dot-product. While there have been many demonstrations of dot-product circuits and, separately, of stochastic neurons, the efficient hardware implementation combining both functionalities is still missing. In my talk I will discuss our recent work on addressing this need. I will first discuss very compact, fast, energy-efficient, and scalable implementation of stochastic dot-product circuits based on passively integrated metal-oxide memristors. The high performance of such circuits is due to mixed-signal implementation, while the efficient stochastic operation is achieved by utilizing circuit’s noise, intrinsic and/or extrinsic to the memory cell array. The dynamic scaling of weights, enabled by analog memory devices, allows for efficient realization of different annealing approaches to improve functionality. I will then discuss several application demonstrations based on passively integrated TiO2-x memristors, including Hopfield networks for solving combinatorial optimization problems, and a Boltzmann machine.
Overcoming Limitations in Evaluation of Memristor Technologies for Neural Networks
Authors: Whitehead William , Veksler Dmitry , Bersuker Gennadi
Published: December 2020

Intrinsic switching characteristics of two resistive memory technologies, hafnia (RRAM)- and carbon nanotube (CNT)- based, are evaluated with respect to their implementation in deep neural networks (DNN) and possible mitigation approaches to performance degradation. Both RRAM (poly-crystalline metal oxide) and NRAM (fabric of interlocking matrix of CNTs) operations employ modulations of the conductive paths through a semi-isolating layer sandwiched between electrodes. RRAM and NRAM characteristics demonstrate similar features when evaluated employing commonly used switching times > 100 ns: variability of the weights update values linked to stochasticity of the switching processes, asymmetric weight change, and memory update saturation.

Though materials are different, the observed undesirable trends have a common cause that is excessive energy released within the duration of switching operations: it induces gradual changes in the material structure surrounding conductive paths. Such changes, in turn, affect temperature distribution and related structural rearrangements during the subsequent switching. In particular, memory update saturation reflects on exhaustion of the number of structural blocks (atoms, CNTs) available for expanding the conductive path under specific used operation conditions. As follows, by reducing energy released in each operation we may suppress excessive structural changes, hence related performance degradation features. Indeed, under sub-ns switching conditions relevant to high-frequency circuitry operations, both RRAM and NRAM demonstrate significant performance improvement making them promising for DNN implementations.
Resistance Change Behavior Under Voltage Pulses at Reset Process of Ti/HfO2/Au-Reram with Various HfO2 Layer Conditions
Authors: Morimoto Masahiro , Hatanaka Rintaro , Yorioka Takuya , Shimizu Tomohiro , Ito Takeshi , Shingubara Shoso
Published: December 2020

1. Introduction

Recently the application of ReRAM to artificial synapse has been paid much attention in the field of neuromorphic computing([1],[2]). For the implementation of neuromorphic synaptic devices, gradual resistance change response by applying voltage pulse trains is required. We investigated the condition of Ti/HfOx/Au-ReRAM to possess gradual resistance change by changing sputtering conditions of HfOx layer.

2. Sample preparation and experiments

Figure 1 shows the structure of Ti/HfOx/Au device. For deposition of HfOx film, we used reactive sputtering with Ar and O2 mixture gases. At this time, we adopted two types of Ar/O2 flow ratios, which is (1)7.8 : 15.6 sccm and (2)7.8 : 2.0 sccm. These devices have a bipolar memory characteristic. SET occurs when a positive voltage is applied, and RESET occurs when a negative voltage is applied. We measured resistance change of RESET process of the ReRAM device with applying DC voltage pulses.

3. Results and discussion

Figure 2 shows resistance change behavior with applying voltage pulses at RESET process. Pulse width is 1.0 uA and pulse period is 2.0 uA. Figure 2(a) is the resistance change behavior of device (1) with constant amplitude voltage pulses. Resistance change was rarely observed when pulse amplitude is -1.5 V, however, gradual resistance change more than 1 digit was observed when pulse amplitude is -1.6 V. Moreover, when we applied -1.7 V, gradual resistance change more than 2 digits within 40 pulses was observed. In the case of this device, resistance change was ideal for the use of neuromorphic synaptic device. Figure 2(b) is the results of device (2). Gradual resistance change was observed after 60 pulses when pulse width is -1.5 V. However resistance increased more than 3 digits after a few pulses were applied when pulse amplitudes were -1.6 V and -1.7 V. In the case of device (2), resistance change is almost binary and it is not suitable for use of artificial synapse device. The difference of resistance change behavior may come from spatial variation of oxygen vacancies. For further understanding of the mechanisms, we will evaluate the difference of atomic composition of HfOx layer by XPS and XRD.
(Invited) Computing with Memristive Devices and Arrays
Authors: Yang J. Joshua
Published: December 2020

Memristive devices have become a promising candidate for unconventional computing due to their attractive properties(1). The computing can be implemented on a Resistive Neural Network (ResNN) with memristor synapses and neurons or a Capacitive Neural Network (CapNN) with memcapacitor synapses and neurons(2).

For ResNNs as computing accelerators, we have built a dot-product engine based on a 128 x 64 1T1R crossbar array using traditional non-volatile memristors with 64 stable analog resistance levels(3). With such computation accelerators, we have demonstrated efficient inference and learning with traditional Machine Learning algorithms(4-7), which is expected to significantly improve the speed and energy efficiency of neural networks.

For ResNNs beyond accelerator applications, we developed diffusive memristors(8)with diffusion dynamics that is critical for neuromorphic functions. Based on the diffusive memristors, we have further developed artificial synapses(8)and neurons(9)to more faithfully emulate their bio-counterparts. We then integrated these artificial synapses and neurons into a small neural network, with which pattern classification and unsupervised learning have been demonstrated(9).

For CapNNs, we have developed pseudo-memcapacitive devices based on the diffusive memristors. Capacitive synapses and neurons enabled by these memcapacitive devices have been developed and used to form a fully integrated CapNN(10), which can implement spiking signal classification and Hebbian-like learning.

Z. Wanget al., Resistive switching materials for information processing. Nature Reviews Materials
5, 173–195 (2020).

Q. Xia, J. J. Yang, Memristive crossbar arrays for brain-inspired computing. Nature materials
18, 309-323 (2019).

C. Liet al., Analogue signal and image processing with large memristor crossbars. Nature Electronics
1, 52-59 (2018).

Z. Wanget al., Reinforcement learning with analogue memristor arrays. Nature Electronics
2, 115-124 (2019).

Z. Wanget al., In situ training of feed-forward and recurrent convolutional memristor networks. Nature Machine Intelligence
1, 434-442 (2019).

C. Liet al., Long short-term memory networks in memristor crossbar arrays. Nature Machine Intelligence
1, 49-57 (2019).

P. Linet al., Three-dimensional memristor circuits as complex neural networks. Nature Electronics
3, 225–232 (2020).

Z. Wanget al., Memristors with diffusive dynamics as synaptic emulators for neuromorphic computing. Nature Materials
16, 101-108 (2017).

Z. Wanget al., Fully memristive neural networks for pattern classification with unsupervised learning. Nature Electronics
1, 137-145 (2018).

Z. Wanget al., Capacitive neural network with neuro-transistors. Nature Communications
9, 3208 (2018).
Effects of ITO Electrode on Electrochromic Properties of WO3 Thin Films in Visible and Near-Infrared Region
Authors: Ayauchi Yasuhiro , Abe Yoshio , Kawamura Midori , Kim Kyung Ho , Kiba Takayuki
Published: December 2020

Indium tin oxide (ITO) thin films have been incorporated into electrochromic (EC) devices as a transparent conductive film. It is highly desirable that EC devices applied to a smart window can control both visible and infrared light, because infrared light accounts for about half of the energy of sunlight. In this study, we fabricated WO3 thin film on ITO thin films with different sheet resistances and studied their EC properties in visible and near-infrared region.

ITO thin films deposited on glass were used as substrates. Tungsten oxide (WO3) thin films were deposited using RF magnetron sputtering system. Crystal structure was characterized by X-ray diffraction. Optical properties of the prepared samples were measured using UV-VIS-NIR spectrophotometer. The electrical resistance of the ITO thin films was measured by a four probe method. EC properties of the WO3 thin film were measured by cyclic voltammetry (CV) and chronoamperometry (CA) in a 0.01 M H2SO4 aqueous electrolyte.

The transmittance of the high sheet-resistance ITO thin film (177 Ω/sq) decreased at a short wavelength region below 455 nm. However, it showed high transmittance above 70% in the visible and near-infrared region (455-2050 nm). In contrast, the low sheet-resistance ITO thin films showed high transmittance above 70% in a wavelength region of 335-1350 nm and its transmittance decreased above 1350 nm. The differences of the transmittance in short and long wavelength regions are considered to be caused by the Burstein-Moss effect and Drude model, respectively. Fig. 1 shows the transmittance spectra of the WO3 thin film with a thickness of 200 nm deposited on the ITO/glass substrates measured in air. Lines (1) and (2) show the transmittance spectra of colored and bleached states of WO3 thin film deposited on high sheet-resistance ITO thin film, and lines (3) and (4) show those of colored and bleached states of WO3 thin film deposited on low sheet-resistance ITO thin film. The WO3 thin film deposited on the high sheet-resistance ITO thin film showed a larger transmittance change and higher transmittance in bleached state in the near-infrared region than those of the WO3 thin film deposited on the low sheet-resistance ITO thin film.

From these results, it was found that the selection of appropriate ITO electrodes is very important to obtain good EC property of WO3 thin films in visible and near-infrared region.

Figure 1
Effect of Electrolyte Concentration on the Electrochromic Properties of Tungsten Oxide Thin Films
Authors: Dzakpasu Cyril Bubu , Appiah Williams Agyei , Ryou Myung-Hyun , Lee Yong Min
Published: December 2020

The worldwide energy crisis has drawn the attention of researchers to energy saving, energy-storing, and energy converting devices. Among them, electrochromic devices (ECDs) are being extensively studied to reduce energy consumption. Electrochromism refers to the persistent and reversible change of optical properties by an applied voltage pulse. Electrochromism has been proposed for many applications such as smart windows, rear-view mirrors, electrochromic displays, sensors, etc. The electrolyte is an indispensable component in the ECDs which strongly affects the electrochromic performance. The transport mechanism for the ions depends on the type and components of the electrolyte. Liquid electrolyte has been the electrolyte of choice owing to its high ionic conductivity. However, due to leakage issues, other alternatives such as gel-polymer electrolytes and solid electrolytes are being employed.

In this study, we evaluate the effect of LiClO4 in PC having different concentrations on the electrochromic performance of RF sputtered tungsten oxide (WO3) thin films. The effect of the electrolyte concentration was characterized by cyclic voltammetry (CV), chronoamperometry, potentiostatic measurement, electrochemical impedance spectroscopy (EIS), and transmittance measurements. In addition, mathematical modeling and simulation is adopted to enhance the understanding of the different salt concentration on the electrochemical performance. The model predictions are in good agreement with experimental cyclic voltammogram and transmittance data obtained from a three-electrode cell with WO3, Pt, and Ag/AgCl as the working, counter, and reference electrode, respectively. From the model predictions, the electrolyte with lower salt concentration exhibits higher coloring efficiency while the electrolyte with higher salt concentration exhibits higher switching speed.
Towards Systematic Understandings and Designs of Electrolyte/Electrode Interfaces By Computational Physics Approaches
Authors: Jinnouchi Ryosuki , Kodama Kensaku , Shinohara Akihiro , Kudo Kenji , Kitano Naoki , Suzuki Takahisa , Hasegawa Naoki , Morimoto Yu
Published: December 2020

Reducing the amount of platinum group metals (PGMs) in catalyst layers is one of major issues in developments of polymer electrolyte fuel cells (PEFCs). Obviously, the mass activity of oxygen reduction reaction (ORR) needs to be enhanced to reduce the PGM-loading. Specific surface area of PGMs also needs to be large enough to decrease the local oxygen transport loss near the catalyst surface [1]. In addition, the catalysts need to be stable enough to ensure long-term operations of PEFCs [2]. To satisfy all these requirements, extensive studies have been carried out on explorations of optimal composition, size and shape of PGM nanoparticles [3] as well as desired controls of catalyst and ionomer distributions [4,5], and remarkable progresses have been achieved in past decades. In those studies, atomistic simulations have made significant contributions on revealing mechanisms underlying experimental observations and designing new materials. Electronic structure calculations adopting density functional theory (DFT), for example, have been utilized as a standard tool to predict the catalytic activity since the first successful reproduction of the volcano plot of ORR [6]. Molecular dynamics (MD) simulations using semi-empirical force fields have been successfully utilized to predict structures and transport properties of ionomer, too [7-9]. In this presentation, we will show results of our applications of DFT and MD simulations to cathode catalysts and ionomer thin films on catalyst surfaces [8,10]. After discussing the power of the conventional methods as well as their limitations, we will discuss recent progresses in state-of-the-art machine-learning methodologies [11-14] as a promising theoretical tool to examine complex finite-temperature properties of electrode/electrolyte interfaces.

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Parametric Modeling Study of PEM Fuel Cell with Platinum Group Metal -Free Cathode Catalyst Layer
Authors: Dunsmore Lisa , Litster Shawn
Published: December 2020

Polymer electrolyte fuel cells offer zero-emissions energy conversion but are commercially limited by cost and durability. Stack cost can be reduced by using platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR). PGM-free catalysts have lower volumetric activity relative to PGM catalysts and consequently require thicker catalyst layers. Thicker catalyst layers result in increased oxygen transport resistance and flooding in the cathode, which must be considered to accurately model PGM-free fuel cells and quantify the distinct transport resistance. For this purpose, we have developed a well validated model to elucidate relative contributions to oxygen transport resistance. This two-phase, transient, non-isothermal, channel-to-channel model contains cathode catalyst layer morphology informed by plasma-focused ion beam (P-FIB) SEM. Furthermore, the model includes agglomerate treatment of volumetrically active catalyst primary particles coated by ionomer. The structure of the agglomerate model is well suited for describing the catalyst layers containing metal organic framework (MOF) derived metal-nitrogen-carbon catalysts that are roughly spherical and volumetrically active for ORR. The MOF-derived Fe-N-C catalyst used in this study’s experiments contains atomically dispersed active sites and uniform, tunable primary particle size. In this presentation, we will report on our investigation of the performance sensitivity to a wide variety of operating conditions and MEA design specificiation, including cell temperature, gas pressures, cathode thickness, hydrophobic pore fractions and wettability, catalyst primary particle size, ionomer content, and ionomer equivalent weight. Non-dimensional characteristic parameter analysis on the model results is used to distinguish the transition between proton conductivity limited and oxygen transport limited performance, and indicate critical values that affect the the through plane ultilization of the catalyst. This information and studies on the impact of active site density help us set catalyst-specific targets for meeting fuel cell performance goals. With highly active PGM-free catalysts, we can model the impact of parameter changes on transport with experimental validation.

DOE ACKNOWLEDGEMENT

This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office (FCTO) under award number DE-EE0008076. The authors gratefully acknowledge research support from the Electrocatalysis Consortium (ElectroCat), established as part of the Energy Materials Network under the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, under contract number DEEE0008076.
Hybrid Lattice Boltzmann Agglomeration Method for Modeling Transport Phenomena in Catalyst Layer of Polymer Electrolyte Membrane Fuel Cells
Authors: Satjaritanun Pongsarun , Shimpalee Sirivatch , Hirano Shinichi , Cetinbas Firat , Ahluwalia Rajesh , Zenyuk Iryna V.
Published: December 2020

The Hybrid Lattice Boltzmann Agglomeration Method (HLBAM) is employed to model transport phenomena and electrochemical kinetics in the catalyst layer of a polymer electrolyte membrane fuel cell (PEMFC). This work shows the advantages of using a direct modeling-based HLBAM approach [1], which incorporates the detailed structure of porous/catalyst layers from X-ray CT as well as transport variables related characteristics and effective properties from the hybrid catalyst layer microstructure [2]. The conceptual method of this approach, with the detailed structure of model geometries, is shown in Figure 1. The detailed geometries consist of the gas diffusion layer (GDL), micro porous layer (MPL), catalyst layer (CL), and hybrid CL microstructure. The local transport variables and effective properties from the hybrid microstructure model are used to perform and simulate the electrochemical kinetics inside the detailed structure of CL. HLBAM can also predict and investigate the distribution of transport variables and electrochemical kinetics during cell operation condition. The studies include the prediction of liquid water saturation/evolution, breakthrough pressure, heat transfer, species transport, and electrochemical kinetics inside the porous and catalyst layers relevant to fuel cell operation. The examination of oxygen distribution within the detailed structure is shown in Figure 2. The predictions using HLBAM were compared and validated with a macro-kinetics model and experimental data [1, 3]. This method can expedite the development of porous components in PEMFCs in a cost-effective manner. The HLBAM simulation can assist the optimization of porous structure design and durability as well as provide insights to water management, particularly in the catalyst layer.

References:

P. Satjaritanun, S. Hirano, I. V. Zenyuk, J. W. Weidner, N. Tippayawong, S. Shimpalee, Journal of The Electrochemical Society, 167, (2020) 013516.

F. C. Cetinbas, R. K. Ahluwalia, N. Kariuki, V. D. Andrade, D. Fongalland, L. Smithd, J. Sharman, P. Ferreira, S. Rasouli, Deborah J. Myers, Journal of Power Sources, 344, (2017) 62-73.

S. Shimpalee, P. Satjaritanun, S. Hirano, N. Tippayawong, J. W. Weidner, Journal of The Electrochemical Society, 166, (2019) F534-F543.

Figure 1
Coarse-Grained Molecular Dynamics Simulation of Perfluorosulfonic Acid Dispersion in Mixtures of Water and Ethanol
Authors: Kuo An-Tsung , Nakabayashi Kazuhiro , Urata Shingo , Watabe Hiroyuki , Hommura Satoru
Published: December 2020

Coarse-grained molecular dynamics simulation was used to investigate the aggregation behavior of perfluorosulfonic acid (PFSA) ionomers with equivalent weight (EW) of 844 in the dilute mixed water and ethanol solutions. Four different chain lengths (10, 20, 60, and 200 repeat units) of PFSA were studies. It was found that the shorter (10 and 20 repeat units) ionomers form spherical aggregates, whereas the longer (60 and 200 repeat units) ones form cylindrical-like aggregates in pure water and in the mixed water/ethanol solvents at ethanol contents ≤ 50 wt%. The aggregated structures become loose when ethanol content is increased to 50 wt%. With further increasing the ethanol content, the PFSA ionomers gradually uncoil their aggregated structures and extend the chains in the solution. The variation of aggregate structure with the ethanol content was consistent with our experimental observation. The finding will be useful for designing ionomer dispersed solution for thin film and catalyst ink fabrication.

Figure caption: EW844 PFSA polymers form cylindrical-like aggregates a low ethanol contents and random coil conformations at high ethanol contents.

Figure 1