bluuebunny/crossref_metadata_2025_split huggingface dataset:
Epitaxial Growth of High Quality GaN Films on Lattice Matched Metallic Layers
Authors: Dabiran Amir M , Machuca Francisco , De Indranil , Weiss Robert 
Published: February 2020

In recent years there have been great advances in wide bandgap electronic and optoelectronic devices based on GaN materials. However, problems arising from growth on non-native substrates, such as sapphire or Si, continue to present major hurdles for development of higher performance and lower cost devices. For example, complex layer structures and processes have been developed to successfully mitigate some of the problems arising from mismatch in both lattice constant (LC) and coefficient of thermal expansion (CTE) between GaN and foreign substrates. However, the added complexity and costs reduce the utility of these techniques for many applications. In the past, GaN growth on metallic substrates, buffer layers and interlayers have been studied by few groups for a number of applications including thermal management in high power devices, back contact for vertical-transport devices, backside mirror for light emitters and detector, and as threading dislocation filters. The major obstacles for the growth of high quality GaN films in these studies have been 1) deposition of single crystal metal film with proper orientation on foreign substrates, 2) avoiding strong chemical reactions and interface mixing during the nucleation of GaN on the metal surface, 3) minimizing mismatch in LC and CTE to avoid high dislocation density, stress cracks or sever wafer bowing, and 4) achieving adequate thermal and chemical stability of the GaN/metal/substrate structure for the demanding growth conditions encountered in deposition of III-nitride films for example by hydride vapor phase epitaxy.
In this work we have studied high quality growth of GaN by RF-plasma assisted molecular beam epitaxy (PA-MBE) and metal organic chemical vapor deposition (MOCVD) on high-quality, single crystal refractory metals deposited by plasma sputtering on c-plane sapphire and Si(111) substrates. We will first discuss the deposition and optimization of fully relaxed high quality metal layer structures that are properly oriented and lattice matched for the growth of low defect GaN films on c-plane sapphire and Si(111). We will then compare the nucleation and growth of GaN by PA-MBE and MOCVD on these metal layers. Some initial results such as x-ray diffraction scans for GaN grown on these lattice-matched metal alloys are quite comparable to high quality GaN grown on c-plane sapphire. Finally, we present some examples of improved wide bandgap devices based on GaN films grown on metal layers.

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(Invited) Unraveling the Efficiency Limits of GaN-Based Emitters and the Surprising Connection to Electron Devices
Authors: Speck James 
Published: February 2020

In this talk, we review our recent experiments designed to reveal the fundamental efficiency limits of GaN-based LEDs: (i) electroemission spectroscopy studies of InGaN-based LEDs with cesiated surfaces provide the first direct measurement of Auger electrons that result from an electron-electron-hole (eeh) Auger process. Auger-generated electrons survive transit through the p-GaN by transfer to a low lying side valley. The relationship between the observation of Auger electrons and the onset of droop provides compelling evidence that the internal Auger process is the dominant droop mechanisms [1]; (ii) related above gap photoemission studies on p-GaN with cesiated surfaces demonstrate a low level side conduction band valley (nominally referred to as “L”) ~0.9 eV above Γ [2]; (iii) atom probe tomography studies on InGaN-based LEDs demonstrate that the indium alloy follows a binomial distribution, as expected for a random alloy. 2D drift-diffusion simulations show that the alloy fluctuations are essential to explain the low turn-on voltages of InGaN-based LEDs [3]; (iv) vertical carrier transport studies on majority carrier analogs to LEDs, namely all n-type InGaN/GaN MQWs and GaN/AlGaN/GaN structures confirm the importance of natural alloy fluctuations in vertical transport [4,5].
[1] J. Iveland et al., Phys Rev. Lett. 110, 177406 (2013)
[2] M. Piccardo et al., Phys. Rev. B, in press (2014)
[3] Y.-R. Wu et al., Appl. Phys. Lett. 101, 083505 (2012)
[4] D. Browne et al., in preparation (2014)
[5] M. Fireman et al., in preparation (2014)
(Invited) Fully Porous GaN p-n Junctions Fabricated by Chemical Vapor Deposition: A Green Technology towards More Efficient LEDs
Authors: Carvajal Joan J. , Mena Josue , Bilousov Oleksandr , Martínez Oscar , Jiménez Juan , Zubialevich V.Z. , Parbrook Peter J. , Geaney Hugh , O'Dwyer Colm , Díaz Francesc , Aguiló Magdalena 
Published: February 2020

GaN is an important wide band-gap semiconductor in electronics and optoelectronics. In its porous form is particularly interesting for developing optoelectronic devices with improved efficiency, such as LEDs with enhanced efficiency and sensors with enhanced sensitivity.
Through chemical vapour deposition (CVD) [1], we have shown that it was possible to produce nanoporous GaN without any secondary etching or chemical post-growth treatment in order to generate the porosity, present only on the (0001) face of the material, providing a green approach for the production of porous GaN. These nanoporous GaN particles can be deposited on different substrates, including boron nitride, silicon, sapphire and SiC, obtaining porous particles with a low density of defects, as revealed by the emission arising only from the band-edge of the particles obtained in the cathodoluminescence studies [2]. By using this methodology of growth, we demonstrated the fabrication of low resistivity ohmic Pt and Au metallic contacts on porous n-type GaN by the formation of intermetallic seed layers through the vapour-solid-solid (VSS) mechanism [3]. Also, we have been able to develop p-type porous GaN by doping with Mg, with a charge carrier concentration of the order of 1018 cm-3 [4].
Through the careful selection of the substrate it is possible to produce porous GaN epitaxial layers [5] that allow for the fabrication of high quality partially and fully porous GaN rectifying p-n junctions, through a 2 step CVD process, and show their behaviour as diodes with effective uniform conduction [6]. These porous junctions have potential applications in high brightness unencapsulated LEDs with enhanced light emitting properties and high surface area sensors with improved sensitivity.
[1] Carvajal & Rojo, Crystal Growth Des. 9 (2009) 320
[2] Carvajal et al., Microsc. Microanal. 18 (2012) 905
[3] Bilousov et al., ACS Appl. Mater. Interfaces 4 (2012) 6927
[4] Bilousov et al., Appl. Phys. Lett. 103 (2013) 112103
[5] Bilousov et al., CrystEngComm 16 (2014) 10255
[6] Bilousov et al., ACS Appl. Mater. Interfaces 6 (2014) 17954
Durability Evaluation of PEFC Catalyst Layers through in-Situ SEM/STEM Techniques
Authors: Hayashi Akari , Kitamura Masahiko , Noda Zhiyun , Sasaki Kazunari 
Published: February 2020

Electrocatalysts for PEFC, made of platinum deposited carbon materials, have an import role to govern PEFC performance. Although the initial activity is high enough to be commercialized, their long-term durability should still be improved. Especially, carbon oxidation under stop/start cycles of fuel cell vehicles, where the potential at the cathode becomes as high as 1.5 V vs RHE. In order to check long-term durability of electrocatalysts against carbon oxidation, we need to perform accelerating degradation tests. Potential cycling up to high potential is usually used to accelerate degradation.
In this study, we introduce our method to accelerate degradation of electrocatalysts using Environmental TEM (Hitachi HF-3300). For carbon oxidation, the air is introduced into the specimen chamber, and the specimen holder is heated up to 200 oC to accelerate degradation instead of cycling to high potential many times. In this method, it is possible to observe degradation of electrocatalysts in real-time. In addition, our method makes it possible that both surface and transmitted images are available at the same time through simultaneous SEM and STEM observation.
So far, we have observed a catalyst composed of platinum nanoparticles on Vulcan carbon, and we have found a new phenomenon of platinum particles embedded into Vulcan carbon with almost no agglomeration. However, on graphitized Vulcan carbon surface, agglomeration more occurs, and embedding of platinum is rather suppressed. Since platinum on graphitized carbon have shown much improved durability on a separate experiment using potential cycling, embedding is believed to be a more predominate factor than agglomeration.
We have further tried similar experiments with a sample directly picked up from the cathode catalyst layer of PEFC, which includes Nafion ionomer. Similar to the case without Nafion, platinum is embedded after the acceleration test as shown in Figure 1, even though agglomeration is slightly increased. However, the surface image is somewhat different. We believe that this is due to the existence of Nafion ionmer on the carbon surface.
We have also controlled the atmosphere of observation using hydrogen or humidified air. We will discuss possible degradation mechanisms through the comparison to accelerated degradation derived by potential cycling.

Figure 1
(Invited) Fuel Cell Catalyst Development and Prospects of Refining Business in Tkk
Authors: Matsutani Koichi 
Published: February 2020

For a full-scale spread of FCs, the necessity of reducing Pt on the stack is increasing. I will introduce effort of TKK for reducing Pt loading, it means that how we tried to improve the catalytic activity. In addition, need of precious metal recycling is growing upon commercializing. I will introduce the prospects for the future of the recycling business.
Elimination of Surface Adsorbates on Gadolinium Doped Ceria for Electrochemical Strain Microscopy
Authors: Xu Jie , Li Jiangyu , Adler Stuart B. 
Published: February 2020

Use of solid oxide fuel cells for the conversion of electrochemical energy has a number of advantages over current technologies. These systems offer high efficiencies for energy conversion, a choice of inexpensive hydrocarbon fuels, and environmentally friendly operation. The mechanisms which govern the functionality of these materials may be elucidated using scanning probe microscopy based techniques. However, the formation of surface adsorbates complicates their use. One such technique, electrochemical strain microscopy, quantifies the mechanical response of a surface resulting from the application of an AC bias through a cantilever. We have used this methodology in conjunction with a number of strategies to understand the behavior of gadolinium doped ceria, a widely used electrolyte material, while circumventing the issues that arise at the material surface. Sections of the ceria sample were ablated using a focused ion beam to expose a surface previously shielded from the ambient environment, then subsequently studied with electrochemical strain microscopy. We find that the ablation process produced a surface morphology exhibiting an enhanced electromechanical response similar to polycrystalline ceria at the grain boundaries that has previously been attributed to the space-charge effect. Thermogravimetric analysis, between 20°C and 400°C, revealed a plateau in weight around 200°C and 350°C, suggesting these are points that surface adsorbates may be removed from the electrolyte. We are currently studying the ablated regions at 250°C in a controlled environment to analyze the strain response under these conditions.
(Invited) Platinum Electrochemial Dissolution and Its Consequences for Platinum Electrocatalysis
Authors: Cherevko Serhiy , Geiger Simon , Keeley Gareth , Topalov Angel A. , Zeradjanin Aleksandar R. , Mayrhofer Karl J.J. 
Published: February 2020

While the fact that platinum dissolves has been known for more than a century, its nature is still obscure. Platinum dissolution as a result of an anodic treatment of a platinum electrode was documented as early as at the beginning of the twentieth century in the works of the founder of electrochemical kinetics, Julius Tafel.1
,
2 In the second half of the twentieth century several groups reported on the quantification of platinum dissolved from platinum electrodes during potentio- or galvano-static or potentiodynamic conditions.3-5 Later, platinum dissolution was mainly discussed in relation to platinum catalyst degradation in proton exchange membrane fuel cell (PEMFC) cathodes.6 Recently, we applied a novel setup composed of a scanning flow cell and inductively coupled plasma mass-spectrometer (SFC-ICP-MS) for precise time- and potential-resolved investigation of platinum dissolution under acidic conditions.7 In the current work we are going to show and discuss these and new results obtained using SFC-ICP-MS. Inter alia an effect of pH, potential, temperature, presence of chloride ions or reactive gases on the process of platinum dissolution will be communicated. Besides dissolution of polycrystalline platinum electrode new data on the stability of nanoparticulated carbon-supported platinum catalysts will be shown and a comparison with the bulk material will be made. Based on the obtained experimental results consequences of platinum dissolution on the application of platinum in electrocatalysis of oxygen reduction and other important reaction of platinum electrocatalysis will be discussed.

References

1. K. Müller, Journal of the Research Institute for Catalysis, Hokkaido University, 17, 54 (1969).
2. G. T. Burstein, Corros. Sci., 47, 2858 (2005).
3. A. N. Chemodanov, Ya. M. Kolotyrkin, M. A. Dem'rovskii and T. V. Kudryavina, Doklady. Akad. Nauk. SSSR, 171, 1384 (1966).
4. D. A. J. Rand and R. Woods, J. Electroanal. Chem., 35, 209 (1972).
5. D. C. Johnson, D. T. Napp and S. Bruckenstein, Electrochim. Acta, 15, 1493 (1970).
6. R. M. Darling and J. P. Meyers, J. Electrochem. Soc., 150, A1523 (2003).
7. A. A. Topalov, S. Cherevko, A. Zeradjanin, J. Meier, I. Katsounaros and K. J. J. Mayrhofer, Chemical Science, 5, 631 (2014).
Plasmonic Metal Oxide Nanocrystals and Their Near Infrared Electrochromism
Authors: Milliron Delia J. , Runnerstrom Evan , Llordes Anna , Wang Yang , Dahlman Clayton 
Published: February 2020

Degenerately doped metal oxide semiconductors, like ITO, exhibit plasmonic resonance at near and mid-infrared wavelengths tunable by varying their composition. Nanocrystals of many such materials have now been synthesized and applications are emerging that leverage the responsiveness of their localized surface plasmon resonance (LSPR) to electronic charging and discharging. One such application is electrochromic glass that can dynamically control heat loads in buildings as the plasmonic absorption of the nanocrystals is electrochemically modulated. I will discuss the principles of electrochromic switching in such materials and describe our efforts to integrate plasmonic nanocrystals into full electrochromic devices. Fundamental understanding of the nature of the electronic states giving rise to infrared absorption is also being pursued. For example, near-edge x-ray absorption spectroscopy is illuminating the charge state of the metal cations in as-prepared doped nanocrystals and in electrochemically-charged materials. Localization of charge upon lithium insertion gives rise to visible tinting, useful for controlling daylight intensity in smart windows. Finally, fabrication strategies compatible with flexible substrates are explored, opening the door to low cost manufacturing.
Ultrathin and Nanostructured Organic Layers with on/Off Switching Properties Based on Covalently Grafted Olidothiophenes
Authors: Lacroix jean Christophe 
Published: February 2020

Conducting surface modification by covalent attachment of organic molecules is needed in research fields such as molecular electronics. or smart surfaces. Among several methods leading to immobilisation of organic molecules onto a surface, one of the most frequent reactions is the electrochemical reduction of an aryl diazonium salt. We have used this method with several diazonium salts containing thiophene units. The resulting surfaces were characterized employing electrochemical measurements and X-ray photoelectron (XPS) analysis. The layers have thicknesses between 3 and 20 nm and are strongly attached to the surface as a result of covalent grafting. Unusual electrochemical responses towards several outer-sphere redox couples have been observed. In case of redox couples bearing a redox potential below a threshold potential, a diode-like behaviour was observed whereas for redox potentials superior to this threshold potential, the film becomes conducting and no modification of the electrochemical signal of the redox probes were observed when compared to bare electrodes. The threshold can be easily tuned by the molecular structure of the grafted oligomers. Ultrathin layer junctions based on conjugated oligomers with reversible on/off switching capabilities controled by the redox state of the oligomer have thus been obtained. 1, 2
Such diazonium salts can be easily included in cyclodextrine. We have shown that the electroreduction of the encapsulated diazonium salts in water solution leads to modified surfaces consisting of grafted oligothiophenes and cyclodextrins which can be removed from the surface. Electroreduction of a second diazonium in the generated holes is possible and leads to mixed layers. 3, 4 Finally such films can be nanostructured using Nanosphere lithography and several nanostructured multifunctional layers can be generated using this inexpensive technique. 5
The use of such ultrathin, nanostructured and multifunctional organic layers will be discussed. 6
1 Fave C.; Noel V.; Ghilane J.; Trippe-Allard G.; Randriamahazaka, H., Lacroix J. C. J. Phys. Chem. C (2008), 112(47), 18638
2 Stockhausen V.; Ghilane J.; Martin P., Trippe-Allard, G.; Randriamahazaka, H.; Lacroix J. C. JACS (2009), 131(41), 14920.
3. Santos L.; Ghilane J.; Martin P.; Lacaze, P.-C.; Randriamahazaka, H.; Lacroix J. C. JACS, (2010) 132(5), 1690.
4. Santos L., Ghilane, J.; Lacroix J. C. JACS (2012) 134 , 12, 5476-5479
5. Santos L., Ghilane, J.; Lacroix J. CElectrochemistry Communications (2012) 18, 20-23
6. Martin P.; Della Rocca, M. L. ; Anthore, A. ; Lafarge, P. ; Lacroix J. C. JACS (2012) 134(1), 154-157. Yan, H.; Bergren, A. J.; McCreery, R.; Della Rocca, M. L.; Martin, P.; Lafarge, P.; Lacroix, J.C. PNAS, (2013) 110, 14, 5326-5330.
An Electrochromic Device Based on Prussian Blue, Polymer Matrix Immobilized Viologen, and Ferrocene
Authors: Lu Hsin-Che , Kao Sheng-Yuan , Chang Ting-Hsiang , Kung Chung-Wei , Ho Kuo-Chuan 
Published: February 2020

Viologens attracted much attention on electrochromic applications for its dramatic optical contrast between its dication state (bleached) and cation-radical state (colored). For the stability concern of eletrochromic device (ECD), solution-type viologen based ECD is necessary to avoid instability. Thus, gel-type electrolytes or immobilized viologens are preferred for solving the long-term instability. In this study, viologen was immobilized on the counter electrode of the ECD by a 3D polymer matrix based gel electrolyte. The polymer matrix with immobilized viologen was fabricated by UV-curing method. Prussian blue (PB) was used as the working electrode of this ECD for its good stability and high contrast ratio. A redox mediator ferrocene (Fc) was added into the ECD to balance the charge between the two electrodes. By doing so, the longer bleaching time of the PB/viologen based ECD (~10 s) was significantly shortened. The working principle of this ECD is sketched in Fig. 1. The PB/Fc/Immobilized viologen based ECD was operated between 1.2 V and -0.8 V. Under this potential bias, the ECD exhibited 61.6% transmittance change (DT%) at 615 nm, and a relatively fast bleaching time of 1.86 s compared to other known PB/viologen based ECD1, is shown in Fig. 2. The proposed ECD also shows a tunable multi-electrochromism among bleached (-0.8 V), open-circuit voltage (OCV, no potential bias) and colored states (1.2 V).
REFERENCES
(1) Kuo, T. H.; Hsu, C. Y.; Lee, K. M.; Ho, K. C. Sol. Energy Mater. Sol. Cells 2009, 93, 1755.

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Electrochromic Devices Using Prussian Blue Electrochromic Layer and HPC-Based Electrolyte
Authors: de Assis Lucas Marinho Nóbrega , dos Santos Leandra Pereira , Sabadini Rodrigo César , Pawlicka Agnieszka 
Published: February 2020

Electrochromic devices (ECDs) have been studied for technological application. The ECDs with Prussian blue (PB)/hydroxypropyl cellulose (HPC) electrolyte/CeO2-TiO2 configuration were assembled and characterized. The films of PB were obtained by galvanostatic electrodeposition applying -0.45 μA for 300 s on ITO glass immersed in a solution of 0.05 M K3Fe(CN)6, 0.05 M FeCl3×6H2O and 0.05 M HCl. The electrolyte was prepared by mixing 0.3 g of HPC with 30 mL of acetone and addition of 10 % m/m of LiClO4. To this mixture, polyethyleneglycol (PEG) also was added. The ECDs were assembled by combining the HPC membrane between the counter-electrode consisting of CeO2-TiO2 and the Prussian Blue electrochromic layer. The devices were characterized by chronocoulometry coupled with UV-Vis transmittance, open circuit properties, and voltammetric cycling. The ECD was switched transparent and blue states applying -2.6 and 2.0 V, respectively. The charge density measurements showed an increase from ~-10 to -14 mC/cm2 when the potentials where applied for 15 s and 60 s, respectively. While this potential is applied, we observe a variation of transmittance in the UV-Vis spectra region, revealing that switching the ECD between -2.6 and 2.0 V for 15 s/15 s and 60 s/60 s resulted in 27 % and 42 % of difference between both states. Changes of transmittance in open circuit after applying a potential of -2.6 V for 15 s revealed a decrease from 50% to 27 % in 24 h. The cyclic voltammetry measurements performed at 100 and 200 mV/s showed better resolutions of the oxidation and reduction peaks for higher scan rates. Moreover, it was observed that after 1,000 cycles, the charge density decreased from -10 to -6.9 mC/cm2 and the difference of transmittance between colored and bleached state decreased from 41 to 30 %. The obtained results revealed that Prussian blue with HPC electrolyte are very promising materials for ECD applications. …