bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatiotemporal Patterns Formed on p-Type Silicon Electrodes during Electrochemical Dissolution
Authors: Suzuki Yuya , Urata Tomoko , Fukami Kazuhiro , Kitada Atsushi , Murase Kuniaki
Published: February 2020

Formation of spatiotemporal patterns has been reported in many electrochemical systems [1]. Applications of spatiotemporal patterns to other fields such as materials science and biological science are important for further progress of nonlinear dynamical systems. Since there are lots of parameters for the emergence of the patterns, it is difficult to understand the mechanisms for pattern formation.

Spatiotemporal patterns are often observed in electrochemical systems that show negative differential resistance (NDR) on their current density vs. potential curves. We have found that an S-shaped NDR was observed when a p-type silicon (p-Si) electrode was anodized in an HF solution containing organic solvents. A well-aligned microgroove array was formed after electrochemical dissolution at a potential in the S-shaped NDR. Although the spatial pattern was formed at the S-shaped NDR region, oscillatory instability in current or potential was not observed. In the present study, we report that an oscillatory instability in current is observed at the S-shaped NDR region.

For the electrochemical measurements, p-Si (100) with a resistivity of 1-4.5 W cm was used as the working electrode. Platinum (Pt) rods were used not only as the counter electrode, but also as the indicator electrode. Electrolyte solution was a mixture of aqueous HF (47 wt.%), ultrapure water and methanol (MeOH) with a composition of 5:6:29 in volume. Since the resistance of the solution is significant to estimate the potential applied at the electric double layer, the resistance was measured by impedance spectroscopy. Temperature of the solution was controlled by a water bath.

Figure 1 depicts current density vs. potential (j-U) curves together with current density vs. iR-corrected potential (j-
f) curves measured in the solution at 20°C and 30°C. When using the solution at 20°C, an S-shaped NDR is observed after correcting the iR-drop in the solution. We have confirmed that an array of microgrooves is spontaneously formed at potentials on the NDR branch. No oscillatory instability is observed in the curves. In the solution at 30°C, a fluctuation in current is observed on the j-U curve. After the correction of iR-drop in the solution, a clear oscillatory instability is observed in the region where the S-shaped NDR is detected in the solution at 20°C.

Time developments of current measured at a constant potential in the NDR region was plotted in Figure 2. Clear current oscillations are not observed in the solution at 20°C although the current fluctuates with time. In contrast, a current oscillation whose period is roughly 1 min is observed. This difference suggests that a bifurcation from the Turing to Turing-Hopf occurs by changing the temperature of the solution from 20 °C to 30°C.

[1] K. Krischer, N. Mazouz, P. Grauel, Angew. Chem. Int. Ed.
40, 850 (2001).
Invited: Nanowire Growth Mechanism upon Treatment of Metals with Non-Equilibrium Oxygen Plasma
Authors: Mozetič Miran
Published: February 2020

Recent results on the growth of metal oxide nanowires upon exposure of metallic samples to non-equilibrium oxygen plasma are presented. The first steps in nucleation and growing of the nanowires were studied using pure niobium foils. A strong gradient of sample temperature was applied by heating the sample on one side only and the formation of nanoparticles along the gradient was studied by scanning electron microscopy. Nanowires were found only on the surface already covered with a rather uniform oxide film. First nanowires were observed growing at the cracks between oxide crystallites allowing for a conclusion that the necessary condition for nanowire growth was preferential migration of metallic atoms along the grain boundaries. A higher temperature allowed for higher density of nanowires and formation of bundles was observed at optimal temperature. The hypothesis was confirmed using iron sample with thick oxide films on the surface formed by thermal oxidation. The thickness of the oxide film was roughly 10 micrometers. Bundles of nanowires on the surface of such materials grew only at elevated temperature higher than about 1100 K what was explained by poor migration of iron atoms through the oxide film at lower temperature. The density of nanowire bundles increased with increasing temperature and at high temperature the one-dimensional growth was lost what has been explained by migration of metal atoms throughout the oxide film rather independent from the local structure.
Supercapacitors Based on Propylene Carbonate with Addition of Sulfur Containing Organic Solvents
Authors: Eskusson Jaanus , Jänes Alar , Thomberg Thomas , Lust Enn
Published: February 2020

The widely used mixed electrolytes for supercapacitors are composed of tetraalkyl ammonium salts and mixed solvents such as propylene carbonate (PC), ethylene carbonate (EC) and ethyl methyl carbonate. Cyclic PC and EC have a high dielectric constant, good compatibility with carbon electrodes enabling the dissolution of salts to sufficient concentrations. However, it is highly viscous and gives poor conductivity even at ambient temperatures. When PC or EC is mixed with linear alkyl carbonates, such as diethyl carbonate (DEC), dimethyl carbonate (DMC), or EMC, more rapid ion transport is achieved as a result of lower viscosity and subsequently higher ionic conductivity is observed. The resulting binary or ternary mixed solutions are widely used in commercially available lithium ion batteries. The choice of co-solvent is a critical issue that significantly affects not only the conductivity, but also the electrochemical performance, including the physicochemical properties of solid electrolyte interface (SEI) formation. In recent years, sulfites such as dimethyl sulfite, glycol sulfite, butylene sulfite, and diethyl sulfite have been studied as a film forming and high temperature additives for electrolytes in lithium ion batteries. It should be mentioned that diethyl sulfite (DES) is used as a film forming agent and high temperature additive for electrolytes in lithium ion batteries improving decomposition resistance of the carbonate-based electrolyte. Propylene sulfite (PS) is used as high temperature additive for electrolytes in lithium ion batteries and it improves the decomposition resistance of the electrolyte.

The main aim of this paper is to establish the electrochemical characteristics of the supercapacitors consisting of the two identical mainly microporous titanium carbide derived carbon (TiC-CDC) electrodes in 1 M (C2H5)3CH3NBF4 solutions in propylene carbonate (PC) and mixtures of PC with diethyl sulphite (DES), and propylene sulfite (PS) at different volume ratios. Electrochemical analysis has been made for PC:DES or PC:PS mixtures based electrolytes using cyclic voltammetry, electrochemical impedance spectroscopy, constant current charge/discharge and constant power methods. The limiting capacitance, characteristic time constants and complex power values calculated depend noticeably on the solvent composition and on temperature applied, i.e., on the viscosity and specific conductivity of electrolyte solution used.

Acknowledgements

The present study was supported by The Estonian Centers of Excellence in Science: High Technology Materials for Sustainable Development, ETF Project 9184, European Regional Development Fund Project SLOKT10209T and Project IUT20-13. Authors of the paper would like to thank Ph.D. Tavo Romann for depositing Al on the electrodes.
Nanomechanical Properties of Titanium Oxide (TiO2) Grown by Atomic Layer Deposition
Authors: Mamun M A , Baumgart Helmut , Elmustafa A A
Published: February 2020

Titanium oxide (TiO2) materials experienced increased attention and growth because of their use as electrode material in lithium rechargeable batteries and they also offer excellent photocatalytic properties due to their stability, nontoxicity, and superb photo catalytic activity. However their absorption of the visible light is restricted due to their wide band gap (3.2 eV). One way of improving their absorption capabilities to the visible light is by doping such wide band gaps semiconducting oxides. The N-doped TiO2 films prepared by sputtering showed noticeable visible light absorption at wavelengths of less than 500 nm due to the bandgap narrowing by mixing of N 2p states with O 2p states [1-3]. Many of the properties in these applications are strongly dependent on titanium oxide’s crystal morphology, surface area, and mechanical properties.

TiO2 films of 200, 300, and 500, ALD cycles were deposited on p-type Si (100) substrates using Atomic layer deposition (ALD) technology. These films have been tested for structural, surface morphology, and nanomechanical properties using nanoindentation. The structural and surface properties will be explored using X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). We will discuss the influence of the deposition technique on the structure and properties of the films citing the superiority of the ALD technique that is particularly used in the fabrication of the films that are presented in this study. A field emission scanning electron microscopy (FE-SEM) image is shown in Figure 1.

References

1- R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, Science, 293, 269 (2001).

2- J. Li, Y. Qin, C. Jin, Y. Li, D. Shi, L. Schmidt-Mende, L. H. Gan, and J. H. Yang,bNanoscale, 5, 5009 (2013).

3- C. X. Feng,G. Q.Xu, J. Lv,Q. M. Huang, Z. X. Zheng, andY. C.Wu, J. Electrochem. Soc, 160, H727 (2013).
Vertical Heterostructures of Layered Metal Chalcogenides by Van Der Waals Epitaxy for Solar Energy Conversion and Catalysis
Authors: Jin Song
Published: February 2020

We report a facile chemical vapor deposition (CVD) growth of vertical heterostructures of layered metal dichalcogenides (MX2) enabled by van der Waals epitaxy for the first time. Few layers of MoS2, WS2, and WSe2 were grown uniformly onto microplates of SnS2 under mild CVD reaction conditions and the heteroepitaxy between them was confirmed using cross sectional transmission electron microscopy (TEM) and unequivocally characterized by resolving the large-area Moiré patterns appeared on the basal planes of microplates. Additional photoluminescence peaks were observed in heterostructures of MoS2-SnS2, which can be understood with electronic structure calculations to likely result from electronic coupling and charge separation between MoS2 and SnS2 layers. This work opens up the exploration of novel large-area heterostructures of diverse MX2 nanomaterials as the material platform for electronic structure engineering of atomically thin two-dimensional (2D) semiconducting heterostructures and novel device applications, such as in solar energy conversion. Furthermore, by controlling the nanostructures and polymorphs of group 6 MX2 (M = Mo, W; X = S, Se) materials, we have significant enhanced their catalytic activity in hydrogen evolution reaction for photoelectrochemical water splitting.
Anisotropic Magnetoresistance of Ferromagnetic Conductive Filament in Resistive Switching Memory
Authors: Otsuka Shintaro , Hamada Yoshifumi , Shimizu Tomohiro , Shingubara Shoso
Published: February 2020

A resistive switching (RS) memory has been expected to be a next-generation memory owing to its high density, low power consumption, and high switching speed. The mechanism of the reversible RS phenomenon has been considered in terms of the formation and rupture of conductive filaments (CF). Recently, it has been reported that a ferromagnetic CF was formed in NiO-based RS memory. This suggested the possibility of the new device that has both RS memory and spintronics device functions. The presence of a ferromagnetic CF in the NiO layer was confirmed by measuring the magnetization of the large number of CFs at room temperature using a SQUID magnetometer. Although, it is not yet confirmed that a single ferromagnetic CF is formed by the SET process. The purpose of this study is to understand the physical properties of a single ferromagnetic CF by focusing on magnetoresistance (MR).

We prepared Ni/HfO2/Pt memory device, having ferromagnetic Ni electrode and the oxide of a non-magnetic metal. The fabricated RS memory devices have a cross-bar structure. The crossed electrode size is 1×1 mm2.

RS characteristic was measured with the bipolar operation mode. When current limit is 5 mA, the anisotropic magnetoresistance (AMR) was observed in the low resistance state. The direction of the external magnetic field is in-plane to the substrate, and the current is perpendicular to the magnetic field. The AMR showed a convex behavior with magnetic field, and the MR ratio was approximately 0.15%. It is strongly suggested that AMR came from a ferromagnetic CF formed in the oxide layer. Further discussions will be shown in the workshop.
Non-Melting Phase Change Memory – Topological-Switching RAM (TRAM)
Authors: Ohyanagi Takasumi , Kitamura Masahito , Tai Mitsuharu , Kinoshita Masaharu , Morikawa Takahiro , Akita Kennichi , Takaura Norikatsu
Published: February 2020

Superlattice phase change memory (PRAM) of the repeated films of [Sb2Te3/GeTe], we call this memory "TRAM", Topological-switching RAM, was studied to show some evidences of non-melting features compared to that of the conventional alloy PRAM, e.g. Ge2Sb2Te5. One evidence was very fast switching with the shorter and the rectangular pulses in both SET and RESET. Other evidence was direct observation of the TRAM using TEM after 10 million switching cycles.
Effect of Organic Solar Cell Using Double Cathode Buffer Layers
Authors: Wang Yeong-Her , Ke Jhong-Ciao , Chen Kan-Lin , Tang I-Tseng , Lee Kuan-Wei , Ji Liang-Wen , Huang Chien-Jung
Published: February 2020

The effect of small molecule organic solar cell based on Boron Subphthalocyanine Chloride (SubPc) and fullerene (C60) by using the double cathode buffer layers was studied. The structure of device is indium tin oxide (ITO)/SubPc (9 nm)/C60 (30 nm)/ 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (10 nm)/3,4,910-perylenetetracarboxylic bis-benzimidazole (PTCBI) (0~3 nm)/aluminum (Al). The thickness of PTCBI in device is varied from 1 nm to 3 nm to get the optimal thickness for the better performance. The power conversion efficiency of device was improved from 2.317% to 2.558% by inserting PTCBI of 1 nm between BCP and Al. The enhancement of efficiency is attributed to the shunt resistant of device increasing from 634 Ω-cm2 to 784 Ω-cm2, resulting in an iprovement of the short current density and the fill factor. From the measurement of external quantum efficiency, it can be found that the percentage of light converted into electrons is increased in whole absorption wavelength of device, showing that the function of PTCBI is not as the optical spacer layer. As a result, the PTCBI is served as an electron transport layer which can export the carrier easily and reduce the carrier recombination.
Random Walk Numerical Simulation of Disordered Semiconductor Heterojunctions
Authors: Mandujano-Ramírez Humberto julián , Oskam Gerko , Gonzalez-Vazquez José Pablo , Anta Juan Antonio
Published: February 2020

Many recent advances in novel solar cell technologies are based on charge separation in disordered semiconductor heterojunctions. In this work we use the Random Walk Numerical Simulation (RWNS) method to model the dynamics of electrons and holes in two disordered semiconductors in contact. Calculations are performed by simultaneously moving electrons and holes in two semiconductors in contact, according to Miller-Abrahams hopping rates, also we have implemented the Marcus hopping model. Recombination is taken into account by implementing a tunnelling distance-dependent recombination mechanism across the interface. This way, charge separation for specific heterostructures with energy distributions of surface traps in the two semiconductors involved can be adequately simulated.

We show that several fundamental features of bulk heterojunction (BHJ) and quantum dot (QD) sensitised solar cells can be accounted for by this model. The dependence of the open-circuit voltage on the HOMO energy of the donor material in BHJ solar cells is accurately predicted assuming short recombination distances and a diffusion mechanism of transport. In addition, the variation of the voltage with temperature is adequately reproduced. The linearity and recombination order in electron density for both types of solar cell can be extracted in two ways: (1) the behaviour of the open-circuit voltage as a function of the illumination intensity, and (2) from the variation of the recombination current with respect to the open-circuit photovoltage. The RWNS model in combination with exponential disorder and an activated tunnelling mechanism for transport and recombination is shown to reproduce correctly experimental charge separation. This provides a theoretical basis to study relevant features of novel solar cell technologies. This allows for the interpretation of impedance spectroscopy measurements [1].

References.

1. Mandujano-Ramírez, H. J.; González-Vázquez, J. P.; Oskam, G.; Dittrich, T.; Garcia-Belmonte, G.; Mora-Seró, I.; Bisquert, J.; Anta, J. A. Charge separation at disordered semiconductor heterojunctions from random walk numerical simulations. Phys. Chem. Chem. Phys., 2014, doi. 10.1039/C3CP54237H, in press.
Characteristics of Inkjet-Printed Separators in Graphene-Based Supercapacitors
Authors: Yang Yong Suk , You In-Kyu , Hong Sung-Hoon , Yun Ho-Gyeong
Published: February 2020

Electrochemical capacitors such as a supercapacitor store electrical charge on high-surface-area conducting materials.1,2 Supercapacitors store energy by forming a double layer of electrolyte ions on the surface of conductive electrodes. They are not limited by the electrochemical charge transfer kinetics of batteries and thus can operate at very high charge and discharge rates. Graphene is a promising material for energy storage, especially for high performance supercapacitors. For real time high power applications, it is critical to have high specific capacitance with fast charging time at high current density.3,4

In this study, we prepared the separators of supercapacitors on cellulous films and papers by using an inkjet printing technology. Several different ink materials such as carbon nanotubes and metal oxides were introduced and the influence of the base materials used as separator was also investigated. The supercapacitors with a graphene nanoparticle composite as electrodes showed a specific capacitance of up to 100 F/g. The inkjet printing conditions such as pulse voltages and pressures were carefully controlled to maximize the performance of supercapacitors. The interface topologies and electric properties of the so formed supercapacitors were investigated by a scanning electron microscope and the measuring system of charge-discharge cycles, respectively.

References

1. R. Kotz and M. Carlen, Electrochimica Acta, 45(15-16), 2483-2498 (2000).

2. K. Sahay and B. Dwidevi, J. Elec. Systems, 5, P8 (2009).

3. X. Xie, L. Qu, C. Zhou, Y. Li, J. Zhu, H. Bai, G. Shi, and L. Dai, ACS Nano, 4, 6050 (2010).

4. C. Liu, Z. Yu, D. Neff, A. Zhamu, and B. Z. Jang, Nano Lett., 10, 4863 (2010).
Room-Temperature Defect-Enabled Spin Functionality in GaAs-Based Coumpound Semiconductors
Authors: Chen Weimin M , Buyanova Irina A
Published: February 2020

Semiconductor spintronics is a newly emerging research field that explores the spin degree of freedom, in addition to charge of the electron, for future electronics, photonics and quantum information technology. The success of spintronics relies on our ability to create, maintain, manipulate and detect spin orientation and coherence at room temperature (RT). During recent years, we have witnessed impressive progresses in developments of novel concepts and innovative approaches in these areas. Unfortunately, a vast majority of earlier achievements have been restricted to cryogenic temperatures and with a rather low efficiency, often requiring an external magnetic field. Here, we present our recent work on a new and unconventional approach of RT defect-enabled spin functionality in a non-magnetic semiconductor without requiring a magnetic layer or external magnetic fields. We demonstrated efficient spin filtering in Ga(In)NAs thin films and nanostructures, enabled by Ga self-interstitials, which is capable of generating a remarkably high spin polarization degree (> 40%) of conduction electrons at RT [1]. We also provided the first experimental demonstration of an efficient RT spin amplifier based on defect engineered Ga(In)NAs with a spin gain up to 2700%! [2] Such a spin amplifier is shown to be capable of amplifying a fast-modulating input spin signal while truthfully maintaining its time variation of the spin-encoded information. By taking advantage of the spin amplification effect, we showed that Ga(In)NAs quantum structures can be employed as efficient RT optical spin detectors, with spin detection efficiency well exceeding 100%. Combining the spin-filtering effect and hyperfine coupling, we further achieved the first realization of RT nuclear spin hyperpolarization in semiconductors via conduction electrons [3], relevant to practical applications of future solid-state quantum computation using nuclear spin qubits and in highly sensitive magnetic resonance imaging. We also demonstrated the potential of such spin engineering to significantly improve performance of spin light-emitting devices and efficiency of photovoltaic devices. Such approaches could potentially provide an attractive, alternative solution to the current and important issues on RT spin-functional semiconductors for future spintronics and spin-photonics.

[1] X.J. Wang et al. Nature Materials 8, 198 (2009)

[2] Y. Puttisong et al. Adv. Materials 25, 738 (2013).

[3] Y. Puttisong et al., Nature Communications. 4, 1751 (2013).
The Role of Passivation Layer during Thermal Annealing for Oxide Semiconductor Thin Films
Authors: Hwang Chi-Sun , Park Sang-Hee Ko , Cho Sung-Heang , Ryu Min Ki , Oh Himchan , Lee Su Jae , Yang Jong-Heon , Byun Chunwon , Park Jonghyurk , Cho Kyung Ik , Chu Hye Yong
Published: February 2020

Oxide TFTs have been considered as a good candidate for the application to backplane device for high quality TFT-LCD and large area OLED panel. It is well known that the passivation play the very important role for the development of the high quality oxide TFTs. The criteria for good passivation layer can be a barrier for environmental factors, not having doping source during thermal annealing, and playing a role of curing the defective interface.

The evolution of the resistance of active films during thermal annealing was monitored under the split of passivation layers and active layers. ALD deposited Al2O3 layer and PECVD SiO2 layer were applied for the passivation layer and sputter deposited Al-doped InZnSnO and InGaZnO were applied for the active layer.

It is found that Al2O3 passivation layer supplies dopants to the active interface (possibly H) during high temperature annealing and the side wall of AIZTO pattern contains many defects, which become dopants during high temperature annealing through detailed measurement of resistance of patterned oxide thin films with various sizes.

It is suggested that PECVD SiO2 with proper pre-treatment (possibly N2O plasma treatment) will be best candidate for the passivation layer in the oxide TFTs, especially in the case of oxide TFTs with high mobility.
Hybrid Nonvolatile Memory Devices with Ferroelectric Copolymers Based on ZnO Nanowire Field Effect Transistors
Authors: Chun Young Tea , Chu Daping
Published: February 2020

Zinc Oxide (ZnO) nanowire (NW) field effect transistors (FETs) have been increasingly popular recently as promising circuitry building blocks. ZnO NWs have a relatively good ohmic contact between the metal electrodes and semiconductor materials, which results in a high device performance and fabrication yield. At the same time, organic thin film transistors (OTFTs) with ferroelectric materials have been researched extensively owing to their nonvolatile memory effect, nondestructive readout and applications to flexible electronics [1, 2]. However, research works on ferroelectric memory devices based NW FETs are still limited [3, 4], while that on their thin film counterparts are relatively widespread [5].

We will first introduce the nonvolatile memory device arrangement based on a ZnO NW FET. Single NW and network NW FET configurations are compared in terms of their associated ferroelectric memory properties. We will show our previous work on the ZnO NWs grown by a chemical vapor transport method on glass substrates [6] together with that by a hydrothermal method on flexible substrates, with the dimensions ranging from 100-300 nm in diameter and up to 5-10 um in length. Our ZnO NW FETs grown on flexible substrates directly shows a high current on/off ratio of up to 105-107with an anticlockwise hysteresis loop present on the transfer curve. The top-gate and bottom-gate structures have been prepared by spin coating a 5 wt% solution of ferroelectric PDVF-TrFE powders in cyclohexane. After spin coating, the ZnO NW FETs with PVDF-TrFE thin films have been thermally cured in a vacuum oven at 150°C for 5 hours. The device showed a memory window of at least 10-20 volts depending on the type of the substrates, which can be tuned by varying the sweep range of the gate voltages.

[1] T. Sekitani et al., Science 326, 1516 (2009).

[2] S.K. Hwang et al., Adv. Mater. 24, 5910 (2012).

[3] D. Yeom et al., Nanotechnology 19, 395204 (2008).

[4] Y.T. Lee et al., Adv. Mater. 24, 3020 (2012).

[5] K.H. Lee et al., Adv. Mater. 21, 4287 (2009).

[6] S. Nedic, et al., Appl. Phys. Letts. 104, 033101 (2014).
Lattice Strain Control Luminescence of Phosphors for LEDs
Authors: Liu Ru-Shi
Published: February 2020

We successfully control luminescence of phosphors such as M2Si5N8:Eu (M = Ca, Sr, Ba) and Ca12Al14O32F2:Eu with lattice strain effects. we show that size-mismatch between host and dopant cations tunes photoluminescence shifts systematically in M1.95Eu0.05Si5−xAlxN8−xOx lattices, leading to a red shift when the M = Ba and Sr host cations are larger than the Eu2+ dopant, but a blue shift when the M = Ca host is smaller. A local anion clustering mechanism in which Eu2+ gains excess nitride coordination in the M = Ba and Sr structures, but excess oxide in the Ca analogues, is proposed for these mismatch effects (Fig. 1). Furthermore, the Ca12Al14
-
zSizO32+zF2
−
z:Eu revealed a crystal chemistry approach to reduce Eu ions from 3+ to 2+ in the lattice which replacing Al3+−F− by the appreciate dopant Si4+−O2
−
is adopted to enlarge the activator site that enables Eu3+ to be reduced (Fig. 2). Photoluminescence results indicated that as-synthesized phosphors display an intense blue emission peaking at 440 nm that was produced by 4f−5d transition of Eu2+, along with the intrinsic emission of Eu3+ under UV excitation. According to these results, we demonstrate that optical and other properties that are sensitive to local coordination environments.
Optical Spectroscopy of Ce3+, Eu3+ and Pr3+ in (M2+)9M3+(PO4)7 (M2+=Ca, Sr) (M3+=Al, Ga, In, Sc, Y, Lu, Gd, La)
Authors: Camardello Sam Jospeh , Srivastava Alok , Toscano Paul
Published: February 2020

Optical properties of Ce3+, Pr3+ and Eu3+ in the compound (M2+)9M3+(PO4)7 (M2+=Ca, Sr) (M3+=Al, Ga, In, Sc, Y, Lu, Gd, La) are reported and discussed. The M3+ ion is octahedrally coordinated in this structure. The crystal field splitting, the centroid shift and the Stokes shift of the Ce3+ ion are reported in these compounds (Figure 1). The optical data is connected with the structural aspects of the host lattice and compared with those that have been reported in the literature.
Comparative Systematic Study of Covalent Effects for the Cr3+-, Mn4+- and Ni2+-Doped Optical Materials
Authors: Srivastava Alok M , Brik Mikhail G.
Published: February 2020

The Cr3+, Mn4+ and Ni2+ ions are widely used transition metal dopants. Laser generation was obtained on the 2Eg-4A2g spin-forbidden emission transition in a number of crystals and ceramics containing trivalent chromium; the same transition of the Mn4+ ions is used for getting red emission; the 1Eg-3A2g spin-forbidden transition of the Ni2+ ions is also used for lasing in the infrared region.

In the present work we review the spectroscopic properties of the Cr3+, Mn4+ and Ni2+ ions in a number of solids. The crystal field strength Dq, Racah parameters B, C and position of the first excited state were all tabulated.

In an attempt to quantify the collected information, we used the model recently developed by us (1-3), which allows to link together the energy of the spin-forbidden 2Eg-4A2g transition of the Cr3+ and Mn4+ ions or the 1Eg-3A2g transition of the Ni2+ ions and covalent effects in the considered crystals. To rationalize the variation of energy of these spin-forbidden transitions, we introduced a non-dimensional parameter

ß1=((B/B0)2+α(C/C0)2)1/2 , (1)

where the subscript “0” refers to the values of the Racah parameters for a free ion, and B and C are the Racah parameters for the same ion in a given crystal. The value of α was 1 for the Mn4+ and Ni2+ ions, and 1.5 for the Cr3+ ions. An advantage of our model is that it takes into account reduction of both Racah parameters B and C.

Fig. 1 shows that the energy of the Ni2+
1Eg→3A2g transition is linearly related to β
1 value (3). This indicates that any description of the nephelauxetic effect for the Ni2+ containing hosts must consider both Racah parameters B and C; the dashed lines in Fig. 1 mark the ±489 cm-1 corridor from the central fit line (this value is the rms deviation of the data points).

Figs. 2-3 depict dependence of the Cr3+ and Mn4+
2Eg→4A2g transition on the β
1 parameter, respectively Again, a linear trend can be seen clearly. The ±σ range (where σ stands for the rms deviation) is shown in both figures. The reason for some data points to deviate from the straight line can be related to the difficulties of unambiguous experimental identification of the zero-phonon lines, which can be hidden by prominent vibronic progressions. Still the maximal difference between the fitting line and mostly deviated data points is a few hundred wave numbers, which is within a typical range of the phonon frequencies for the majority of optical materials.

Further studies of the transition metal doped materials are under way; the main aim of these studies is to elucidate the common and different features in the behavior of the spin-forbidden transitions and covalency effects experienced by the transition metal ions in solids.

REFERENCES

1. A.M. Srivastava, M.G. Brik, J. Lumin.
132, 579, (2012).

2. M.G. Brik, A.M. Srivastava, ECS J. Solid State Sci. Technol.
2, R148, (2013).

3. M.G. Brik, A.M. Srivastava, N.M. Avram, A. Suchocki, J. Lumin.
148, 338, (2014).
Self-Propelled Motion of Oil Droplets on Au Electrode during Sn Electrodeposition
Authors: Shiono Tetsuaki , Mukouyama Yoshiharu
Published: February 2020

INTRODUCTION

It was reported that macroscopic contact angles of an oil droplet put on an electrode surface decreased during electrodeposition of metals, such as Sn, Pb and Sb [1]. It was also reported that the difference in the contact angle between the front and rear sides of the droplet caused self-propelled lateral motion of the droplet [1]. This motion has attracted much interest from the viewpoint of the emergence of mechanical motion induced by chemical or electrochemical reactions.

Little has been reported about the factors affecting the velocity of the motion. It has only been reported that the velocity increases with a negative shift in the electrode potential [1]. In this work, we studied the factors affecting the velocity and the mechanism of the motion.

EXPERIMENTAL

The self-propelled motion of a nitrobenzene droplet on an Au ring electrode was observed during Sn electrodeposition (Sn2+ + 2e- → Sn) in H2SO4 + 0.01 M SnSO4. The volume of the droplet was about 1.0 μL. Figure 1 shows schematic of the experimental setup. In order to study the dependence of the velocity on the density of the aqueous electrolyte, K2SO4was added to the electrolyte to increase the density.

RESULTS and DISCUSSION

We studied the dependence of the velocity on the electrode potential when the concentration of H2SO4was 0.5 M. As shown in Figure 2, the droplet did not move, i.e., the velocity was zero, in the potential region above ca. -0.44 V, though the Sn electrodeposition occurred. On the other hand, the droplet moved in the potential region below ca. -0.44 V where both the Sn electrodeposition and hydrogen evolution reaction (HER) occurred. The velocity increased with a negative shift in the electrode potential.

In the potential region where the droplet moved, the Sn electrodeposition was limited by the mass-transport of Sn2+ions to the electrode surface, and hence the rate of electrodeposition was constant independently of the potential. We, therefore, think the velocity increase with a negative shift in the potential is probably attributed to the increase in the rate of the HER. Based on this idea, we suggest that the droplet motion is driven by the interaction between the Sn electrodeposition and the HER.

We also studied the dependence of the velocity on the density of the aqueous electrolyte. As shown in Figure 3, the velocity observed at -0.66 V vs. SHE increased as the concentration of K2SO4increased. That is, the velocity increased as the density increased. The buoyancy of the droplet increased with the increasing density, which probably led to the increase in the velocity.

This work is partially supported by the Research Institute for Science and Technology of Tokyo Denki University under Grants Q12E-02.

REFERENCES

[1] S. Nakanishi, T. Nagai, D. Ihara, Y. Nakato, Chemphyschem, 9 (2008) 2302-2304.
New Computational Insights to Describe the Oxygen Reduction Reaction on Graphene Containing Multiple Nitrogen Functional Groups
Authors: Ortiz-García H. , Vazquez-Arenas J. , Galano Annia , González Ignacio
Published: February 2020

The development of new cathodes for low temperature fuel cells has spurred the research of low cost and durable materials, particularly non-noble materials displaying a similar performance as Platinum [1]. To this concern, metal-free nitrogen doped graphene containing various nitrogen functional groups (quaternary, pyrollic, and pyridinic species) have been found to facilitate the electrocatalysis of Oxygen Reduction Reaction (ORR) as in traditional Pt-based catalysts [2]. Atomistic simulations have been conducted to describe the ORR mechanism on different carbonaceous materials. However, these analysis have mostly been dedicated to Platinum (Pt)-based catalysts, and single nitrogen groups on Carbon nanotubes (CNTs) [3]. Thus, this study is oriented to account for the quantum mechanics interactions arising during the ORR on N-doped graphene structures containing pyridinic, pyrollic and quaternary species. Density functional theory is utilized to compute the Gibbs energy of the adsorbed intermediates and chemical species (e.g. H, OOH, H2O2, OH, O, and O2) formed during the course of the ORR mechanism. The calculations evaluate the scale-up of the cluster in order to screen out local effects due to dimensions, and the reactivity of the aforementioned species on all possible catalytic sites (refer to positions a,b,c, adjacent to C1, C2, PR, PD in figure 1a). It was found that H, OOH, OH, O, and O2 preferentially chemisorb or physorb on the quaternary site C1-a, whereas H2O2 species reacts favorably on C2-b.

Keywords: ORR, DFT, Pyridinic, Pyrollic, and Quaternary Species.

[1] Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L., Science
2009, 323, 760.

[2] Biddinger, E.; Deak, D. v.; Ozkan, U., Top. Catal.
2009, 52, 1566.

[3] Hyman, M. P.; Medlin, J. W., J. Phys. Chem. B
2006, 110, 15338–15344.
Synthesis of Mesoporous Ordered LiFePO4 /C Structures for Cathodes of Li-Ion Batteries
Authors: Palacios Enriquez Ana Y. , Vazquez-Arenas J. , Vera-Robles L.I. , González I.
Published: February 2020

The LiFePO4 phase has become a very promising cathode to be utilized in Li-ion batteries, as a result of its stability during intercalation, low toxicity, low cost and theoretical capacity of 170 mAhg-1. Unfortunately, this compound possesses low ionic and electronic conductivities resulting in a low performance of the battery. Different strategies have been proposed to improve the conductivity of the material (e.g. doping, mixing with carbon particles, finer particles). Recently, the development of a structured mesoporous LiFePO4/C has enabled to attain a larger specific capacity of 166 mAhg-1 and longer lifetime. However, the maximum capacities of this compound have not been investigated at all. Therefore, this study is oriented to improve the synthesis process of a structured mesoporous LiFePO4/C phase, in order to enhance the conduction and diffusion mechanisms. A structured mesoporous silica compound (SBA–15) is used as template for the fabrication of a mesoporous carbon structure, where the LiFePO4 phase is impregnated. This material is then compared (performance of the LiFePO4 in the battery) with that prepared using an alternative and faster synthesis route using activated carbon with large surface area.
Characteristics and Performance of LiMn0.3Co0.3Ni0.3Ti0.1O2 Cathode Materials
Authors: Elong Kelimah , Kamarulzaman Norlida , Rusdi Roshidah
Published: February 2020

Single phase LiMn0.3Co0.3Ni0.3Ti0.1O2 materials are prepared using a self-propagating combustion method. The structure and morphology of the materials were characterized using X-Ray powder diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and X-Ray photoelectron Spectroscopy (XPS). The electrochemical performances of the materials were characterized by means of galvanostatic charge-discharge test on the fabricated cells. XRD results showed that the materials are impurity-free and single phase with well ordered hexaganol layered structure of R-3m space group. The compounds annealed at 700 °C at different annealing times showed variations in discharge capacities. The discharge capacities are between 143 and 144 mAhg-1. The compound annealed at 700 °C for 24 h exhibits the highest first cycle capacity of 143.72 mAhg-1 over the voltage range of 2.5 to 4.2 V. The 30th cycle, however, revealed that the materials annealed for 72 h shows the highest specific discharge capacity of 136 mAhg-1. The capacity fading is only about 4% compared to 23% for the 24 h sample. The longest annealing time of 72 h suggests that highly crystalline LiMn0.3Co0.3Ni0.3Ti0.1O2 materials have more stable crystal structure and exhibit good cycling behaviour compound to the 24 h annealed sample. The results suggest that the self-propagating combustion method is a promising technique for the preparation of LiMn0.3Co0.3Ni0.3Ti0.1O2 cathode material for lithium-ion batteries.
LiCoPO4 Based High Energy Li-Ion Cathodes
Authors: Allen Jan L. , Wolfenstine Jeff , Allen Joshua L. , Jow T. Richard
Published: February 2020

There is an ongoing need for higher energy batteries that operate with the highest safety margin. LiCoPO4 is a 4.8 V cathode material with potentially better abuse tolerance and energy density than the current state of art Li-ion battery chemistries. The intense development of high voltage electrolytes1 and strategies that improve the cycle life2, discharge capacity2 and rate capability3have increased the interest in this material.

This paper will review our recent efforts on LiCoPO4 electrode development for improving the energy and abuse tolerance of Li-ion batteries. We will discuss synthesis methods, transport measurements,3 substitutional chemistry of LiCoPO4 and performance enhancements that result from reformulating the electrolyte.

References

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J. L. Allen, T. R. Jow, J. Wolfenstine, J. Power Sources 2011, 196, 8656.

J.L. Allen, T. Thompson, J. Sakamoto, C.R. Becker, T.R. Jow and J. Wolfenstine, J. Power Sources 254, 204 (2014).
Graphite-Based Negative Electrodes: Correlation of Electrode Expansion and Cyclability
Authors: Bünzli Christa , Cericola Dario , Hucke Thomas , Spahr Michael , Novák Petr , Gomez-Camer Juan Luis
Published: February 2020

Silicon is the most attractive candidate to upgrade carbon as negative electrode material in lithium-ion batteries, due to its large specific charge and low reaction potential. However, the volume changes of silicon upon lithiation and delithiation lead to mechanical stress and poor performance, hindering the commercial use of silicon particles in lithium-ion battery negative electrodes. Many attempts to reduce the volume changes and achieve stable cyclability for silicon electrodes involve the use of carbon and/or graphite as electrochemically active matrix. Here we study the effect of silicon addition to graphite based electrodes and explore the differences in electrochemical response by means of in situ electrochemical dilatometry.

Electrodes based on TIMCAL graphites, TIMREX® KS6 and TIMREX® SLP30, and silicon nanoparticles have been prepared by a simple mixture procedure and coated on Cu foil by standard doctor blade coating technique. Electrochemical performance and cycling stability of the composite electrodes was explored in coin type cells cycled in EC:DMC 1:1w:w, 1M LiPF6 electrolyte using metallic lithium as counter electrode. Different parameters such as the nature of the binder, the effect of graphite morphology, and the use of electrolyte additives are studied in long term cycling experiments.

The expansion of electrode layers was measured in a special dilatometry setup using three-electrode configuration, with both counter and reference electrodes of metallic lithium. A similar cycling program was used to explore the electrode’s expansion, however only during the first few cycles. Differences observed in the expansion of the composite electrode layers help us to understand the distinct electrochemical behaviour of each graphite based system, with and without silicon particles.
Electrochemical and in-Situ Investigation of Quaternary Lithium Transition Metal Fluorides in Li-Ion Batteries
Authors: Lieser Georg , Dräger Christoph , Glatthaar Sven , Geßwein Holger , de Biasi Lea , Indris Sylvio , Ehrenberg Helmut , Binder Joachim R.
Published: February 2020

During the last decade Lithium metal oxides or polyanionic compounds containing two or more different transition metal ions (e.g. LiNi0.5Mn1.5O4 or LiNi1-x-yCoxMnyO2) are in the focus of research and commercial interest as positive electrodes for Li-ion batteries. Lithium transition metal fluorides are very promising materials compared to common oxide materials with corresponding electrochemically active cations because the more electronegative fluorine atoms increase the redox potential leading to a higher specific energy. However, no reports are given about the electrochemical properties of quaternary lithium transition metal fluorides as positive electrode. In this study a novel sol-gel synthesis route was applied to synthesize several quaternary lithium transition metal fluorides LiMFeF6 (M2+ = 3d transition metal) without toxic chemicals like HF, LiF or F2(gas). After the structural and morphological characterization the fundamental electrochemical properties are characterized. It could be shown that up to 1 eq. Lithium can be inserted fully reversible into the LiMFeF6 host structure with a notable cycling stability and a remarkable rate performance. Furthermore an in-situ x-ray powder diffraction and x-ray absorption investigation reveals that the host structure is an insertion material. The electrochemical active redox couple Fe3+/2+ was confirmed by Mößbauer analysis.
Visualizing Longitudinal and Transversal Strain Effects in High-Energy NMC Cathodes Induced By Intense Calendering
Authors: Schünemann Jan-Hinnerk , Petri Ralf , Kwade Arno
Published: February 2020

One of several challenges in improving the performance of lithium-ion batteries is the enhancement of energy density. Apart from the development of ever better active materials, considerable potential for improvement can be found in the complex manufacturing process. The manufacturing of lithium-ion battery cells exhibits a long, mostly sequential process chain with numerous interdependent parameters and quality-determining characteristics. Among slurry mixing and coating, calendering is the most challenging process step and considered of superior importance to the quality of the electrode. Calendering refers to compressing the porous coating layers of the electrode to a predetermined thickness. To obtain the desired energy density and to prepare a homogeneous electrode thickness it is necessary to densify the coating of the electrodes. In this, more electrochemically active material can be accommodated in every unit of electrode volume. In essence, this defines the potential energy density of the cell. As for high-energy battery cells energy density is one of the most important optimization criteria to achieve ambitious requirements for automotive applications calendering is of crucial importance.

However, compression of the electrode is not merely beneficial to the overall performance of the cell. It is a trade-off between achieving high energy density and, on the contrary, damaging the electrode’s structure by applying high mechanical forces. An effect rarely taken into account is mechanical strain induced to the electrode’s metal foil underlying the coating, which can lead to serious issues in manufacturing and malfunction of the battery cell.

This paper addresses macroscopic changes in electrode’s metal foils resulting from intense calendering procedures of high-energy NMC cathodes. Figure 1 exemplarily depicts an area-resolved longitudinal strain map showing irreversible deformation from calendering. The left green section reflects an uncoated edge of the electrode which is not deformed during calendering. The coated section of the specimen is deformed significantly. The data is derived from before/after comparison of the electrode’s surface, conducted with a non-contact optical 3D deformation measuring system. To minimize undesired strain inducement, multi-calendering strategies are assessed which gently apply loads and at the same time still provide for highly compressed electrodes.