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1. Thermally Stimulated Depolarization Current Measurements on Degraded Lead Zirconate Titanate Films

   https://doi.org/10.1039/d1tc02668  

   Akkopru-Akgun, Betul ; Marincel, Daniel M. ; Tsuji, Kosuke ; Bayer, Thorsten J.M. ; Randall, Clive A. ; Lanagan, Michael T. ; and Trolier-McKinstry, Susan ( October 2021 , Journal of the American Ceramic Society)

   null (Ed.)

   Charge transport mechanisms governing DC resistance degradation in ferroelectric films are influenced by defects, particularly oxygen vacancies. This paper demonstrates that oxygen vacancies migrate in lead zirconate titanate (PZT) films under a DC bias field and contribute to resistance degradation. Model PZT thin films were developed in which the concentration and distribution of oxygen vacancies were controlled via (a) changing the dopant type and concentration from 1%–4% Mn (acceptor) to 1%–4% Nb (donor) or (b) annealing undoped PZT films at varying partial pressures of PbO. The presence of associated (immobile) and dissociated (mobile) oxygen vacancies was distinguished by thermally stimulated depolarization current (TSDC) measurements. The impact of mobile oxygen vacancies on local defect chemistry and associated charge transport mechanisms was explored by electron energy loss spectroscopy (EELS). For Mn-doped PZT films, following resistance degradation, TSDC studies revealed only one depolarization peak with an activation energy of 0.6–0.8 eV; this peak was associated with ionic space charge presumably due to migration of oxygen vacancies. The magnitude of the depolarization current peak increased with increasing degradation times. A similar depolarization current peak attributed to the existence of mobile oxygen vacancies was also observed for undoped and Nb-doped PZT films; the magnitude of this peak decreased as the Nb or PbO contents in PZT films increased. An additional TSDC peak associated with polaron hopping between Ti3+ and Ti4+ was found in both Nb-doped PZT films and undoped PZT films annealed under low PbO partial pressure. Degraded Nb-doped samples exhibited a chemical shift in the TiL2,3 peak to lower energy losses and the appearance of shoulders on the t2g and eg peaks, implying a reduction of Ti cations in regions near the cathode. 

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2. The effect of imprint on remanent piezoelectric properties and ferroelectric aging of PbZr 0.52 Ti 0.48 O 3 thin films

   https://doi.org/10.1111/jace.16367  

   Akkopru‐Akgun, Betul ; Zhu, Wanlin ; Lanagan, Michael T. ; Trolier‐McKinstry, Susan ( March 2019 , Journal of the American Ceramic Society)

   Ferroelectric films suffer from both aging and degradation under high ac‐field drive conditions due to loss of polarization with time. In this study, the roles of defect chemistry and internal electric fields on the long‐term stability of the properties of piezoelectric films were explored. For this purpose, lead zirconate titanate (PZT) films with a Zr/Ti ratio of 52/48 doped with Mn‐ (PMZT) or Nb‐ (PNZT) were deposited on Pt coated Si substrates by the sol‐gel method. It was demonstrated that the magnitude of the internal field is much higher in PMZT films compared to PNZT films after poling in the temperature range of 25‐200°C under an electric field of −240 kV/cm. The development of the internal field is thermally activated, with activation energies from 0.5 ± 0.06 to 0.8 ± 0.1 eV in Mn doped films and from 0.8 ± 0.1 to 1.2 ± 0.2 eV in Nb doped films. The different activation energies for imprint suggests that the physical mechanism underlying the evolution of the internal field in PMZT and PNZT films differs; the enhanced internal field upon poling is attributed to (a) alignment of oxygen vacancy—acceptor ion defect dipoles (,) in PMZT films, and (b) thermionic injection of electron charges and charge trapping in PNZT films. In either case, the internal field reduces back switching, enhances the remanent piezoelectric properties, and dramatically improves the aging behavior. PMZT films exhibited the greatest enhancement, with reduced high temperature (180°C) aging rates of 2%‐3%/decade due to improved stability of the poled state. In contrast, PNZT films showed significantly larger high temperature aging rates (15.5%/decade) in the piezoelectric coefficient, demonstrating that the fully poled state was not retained with time.

    

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3. Nanostructured core–shell metal borides–oxides as highly efficient electrocatalysts for photoelectrochemical water oxidation

   https://doi.org/10.1039/c9nr09818f  

   Lu, Can ; Jothi, Palani R. ; Thersleff, Thomas ; Budnyak, Tetyana M. ; Rokicinska, Anna ; Yubuta, Kunio ; Dronskowski, Richard ; Kuśtrowski, Piotr ; Fokwa, Boniface P. ; Slabon, Adam ( February 2020 , Nanoscale)

   null (Ed.)

   Oxygen evolution reaction (OER) catalysts are critical components of photoanodes for photoelectrochemical (PEC) water oxidation. Herein, nanostructured metal boride MB (M = Co, Fe) electrocatalysts, which have been synthesized by a Sn/SnCl 2 redox assisted solid-state method, were integrated with WO 3 thin films to build heterojunction photoanodes. As-obtained MB modified WO 3 photoanodes exhibit enhanced charge carrier transport, amended separation of photogenerated electrons and holes, prolonged hole lifetime and increased charge carrier density. Surface modification of CoB and FeB significantly enhances the photocurrent density of WO 3 photoanodes from 0.53 to 0.83 and 0.85 mA cm −2 , respectively, in transient chronoamperometry (CA) at 1.23 V vs. RHE (V RHE ) under interrupted illumination in 0.1 M Na 2 SO 4 electrolyte (pH 7), corresponding to an increase of 1.6 relative to pristine WO 3 . In contrast, the pristine MB thin film electrodes do not produce noticeable photocurrent during water oxidation. The metal boride catalysts transform in situ to a core–shell structure with a metal boride core and a metal oxide (MO, M = Co, Fe) surface layer. When coupled to WO 3 thin films, the CoB@CoO x nanostructures exhibit a higher catalytic enhancement than corresponding pure cobalt borate (Co-B i ) and cobalt hydroxide (Co(OH) x ) electrocatalysts. Our results emphasize the role of the semiconductor–electrocatalyst interface for photoelectrodes and their high dependency on materials combination. 

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4. Comparing Pr 3+ and Nd 3+ for deactivating the Er 3+ :  4 I 13/2 level in lanthanum titanate glass

   https://doi.org/10.1364/OME.517790  

   Topper, Brian ; Neumann, Alexander ; Wilke, Stephen K. ; Youngman, Randall E. ; Alrubkhi, Abdulrahman ; Weber, Richard ( April 2024 , Optical Materials Express)

   Erbium lanthanum titanate glasses were prepared by levitation melting for the spectroscopic study of ways to promote the mid-infrared fluorescence. Two series of heavily erbium doped glasses (15 wt%) were prepared with the addition of either Pr³⁺or Nd³⁺in amounts relative to Er³⁺of 0.05, 0.1, and 0.2. Both ions quench the lower Er³⁺laser level with the Pr³⁺doing so more rapidly. Although high co-dopant concentrations result in higher energy transfer, as clearly evidenced in upconversion and downconversion fluorescence measurements, the mid-infrared lifetime also suffers a reduction and, therefore, a balance must be struck in the co-dopant concentration. Lifetime and spectral measurements indicate that, at a fixed relative co-dopant amount, Pr³⁺is more effective than Nd³⁺at removing the bottleneck of the Er^3+ 4I_13/2level. Moreover, consideration of the lifetimes alongside the absorption data of the individual ions indicates that despite the large absorption cross-section of Nd³⁺at 808 nm, the concentration needed to yield more absorbed power than utilizing direct 976 nm excitation of Er³⁺results in unfavorable lifetimes of the mid-infrared transition. In the end, Pr³⁺prevails as the superior co-dopant in terms of the effects on fluorescence lifetimes as well as potential laser system design considerations. In a unique self-doping approach, a reducing melt atmosphere of Ar instead of O₂creates a small fraction of Ti³⁺. In 5Er₂O₃-12La₂O₃-83TiO₂glass, the presence of Ti³⁺quenches the⁴I_13/2emission about 2.6 times more than the⁴I_11/2when lifetimes are compared to an O₂melt environment. As an additional means of increasing the mid-infrared emission, the effect of temperature on the mid- and near- infrared lifetimes of a lightly doped lanthanum titanate composition is investigated between 77-300 K. The mid-infrared lifetime increases by ∼30% while the near-infrared lifetime increases by ∼10%, which suggests in addition to co-doping, active cooling of the gain media will further enhance performance.

    

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5. n‐Doping of a Low‐Electron‐Affinity Polymer Used as an Electron‐Transport Layer in Organic Light‐Emitting Diodes

   https://doi.org/10.1002/adfm.202000328  

   Smith, Hannah L. ; Dull, Jordan T. ; Longhi, Elena ; Barlow, Stephen ; Rand, Barry P. ; Marder, Seth R. ; Kahn, Antoine ( February 2020 , Advanced Functional Materials)

   n‐Doping electron‐transport layers (ETLs) increases their conductivity and improves electron injection into organic light‐emitting diodes (OLEDs). Because of the low electron affinity and large bandgaps of ETLs used in green and blue OLEDs, n‐doping has been notoriously more difficult for these materials. In this work, n‐doping of the polymer poly[(9,9‐dioctylfluorene‐2,7‐diyl)‐alt‐(benzo[2,1,3]thiadiazol‐4,7‐diyl)] (F8BT) is demonstrated via solution processing, using the air‐stable n‐dopant (pentamethylcyclopentadienyl)(1,3,5‐trimethylbenzene)ruthenium dimer [RuCp*Mes]₂. Undoped and doped F8BT films are characterized using ultraviolet and inverse photoelectron spectroscopy. The ionization energy and electron affinity of the undoped F8BT are found to be 5.8 and 2.8 eV, respectively. Upon doping F8BT with [RuCp*Mes]₂, the Fermi level shifts to within 0.25 eV of the F8BT lowest unoccupied molecular orbital, which is indicative of n‐doping. Conductivity measurements reveal a four orders of magnitude increase in the conductivity upon doping and irradiation with ultraviolet light. The [RuCp*Mes]₂‐doped F8BT films are incorporated as an ETL into phosphorescent green OLEDs, and the luminance is improved by three orders of magnitude when compared to identical devices with an undoped F8BT ETL.

    

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