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At least 55 records · Page 3

Controlling Nb(IV) Defects in SrNbO 2 N Oxygen Evolution Photocatalyst by Ammonolysis With Dinitrogen–Ammonia Mixtures

Strontium niobium oxynitride (SrNbO 2 N) is a promising, corrosion resistant semiconductor for the visible light-driven water splitting reaction, a non-photovoltaic pathway to green hydrogen fuel. However, SrNbO 2 N materials made by ammonolysis usually contain Nb 4+ defect states that cause electron–hole recombination. Here, in this work, we demonstrate that such defects can be minimized by synthesizing SrNbO 2 N from metal oxides in a mixed 13%:87% (vol) NH 3 /N 2 atmosphere. According to electron paramagnetic resonance (EPR), SrNbO 2 N made in pure NH 3 contains paramagnetic impurities with g = 2.002 and 2.195, which can be assigned to lattice and surface Nb 4+ defects. These states also cause broad optical absorptions centered at 800 and 1020 nm, respectively, and the lattice defect produces a 1.55–1.63 eV signal in surface photovoltage spectra. The improved SrNbO 2 N contains five times fewer lattice Nb 4+ defects (8.95 × 10 15 cm −3 ), based on the integrated EPR signal intensity, and supports a water oxidation photocurrent of 1.07 mA cm −2 at 1.23 V versus RHE under simulated sunlight and an apparent quantum efficiency of 5.1% at 400 nm during photocatalytic oxygen evolution. Based on earlier results with LaTiO 2 N and BaTaO 2 N, dilution of NH 3 during synthesis appears generally beneficial to transition metal oxynitrides.

electron paramagnetic resonance↗

Nanopinhole-Enabled, Hole-Selective Poly-Si/SioxNy Passivating Contacts on Textured c-Si for Si Solar Cells

The next-generation silicon photovoltaics will be based on passivating electron- and hole-selective contacts with both very low interface recombination and contact resistivities. While the emerging mainstream TOPCon technology has developed excellent electron-selective poly-Si/tunneling SiOx contacts, hole-selective contacts, especially on textured surfaces, have remained a significant challenge. This contribution introduces novel high-performance hole selective poly-Si contacts on pyramid-textured Si, enabled by electrochemically produced hole transport nanopinholes in a 10 nm oxynitride passivating dielectric stack capped by p+ poly-Si. The highly passivating oxynitride layer is produced via atomic intermixing of O and N atoms in the initial SiOx/SiNy layer stack upon thermal annealing. Carrier transport is governed by nanopinhole density and size are tuned by Ag nanoparticle electrodeposition and surface attachment chemistries. This results in passivating hole contact resistivities in the m..omega..-cm2 range, while preserving interface recombination current prefactor around 5 fA/cm2.

14 SOLAR ENERGY↗

Nanoscale Li, Na, and K ion-conducting polyphosphazenes by atomic layer deposition

A key trailblazer in the development of thin-film solid-state electrolytes has been lithium phosphorous oxynitride (LiPON), the success of which has led to recent progress in thin-film ion conductors. Here we compare the structural, electrochemical, and processing parameters between previously published LiPON and NaPON ALD processes with a novel ALD process for the K analogue potassium phosphorous oxynitride (KPON). In each ALD process, alkali tert-butoxides and diethylphosphoramidate are used as precursors. To understand the ALD surface reactions, this work proposes a reaction mechanism determined by in-operando mass spectrometry for the LiPON process as key to understanding the characteristics of the APON (A = Li, Na, K) family. As expected, NaPON and LiPON share similar reaction mechanisms as their structures are strikingly similar. KPON, however, exhibits similar ALD process parameters but the resulting film composition is quite different, showing little nitrogen incorporation and more closely resembling a phosphate glass. Due to the profound difference in structure, KPON likely undergoes an entirely different reaction mechanism. Furthermore, this paper presents a comprehensive summary of ALD ion conducting APON films as well as a perspective that highlights the versatility of ALD chemistries as a tool for the development of novel thin film ion-conductors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Ammonolysis with N 2 -diluted NH 3 suppresses Ta( IV ) defects in BaTaO 2 N and enhances photocatalytic water oxidation

BaTaO 2 N stands out among oxynitride photocatalysts because of its ability to capture visible light and to drive the photoelectrochemical water oxidation reaction. However, its solar energy conversion performance is limited by electron–hole recombination at Ta( IV ) defects in the material. These defects are formed by overreduction of the Ta(v) oxide precursor by excess ammonia under the high temperature conditions during ammonolysis. Here we show for the first time that Ta( IV ) defect concentrations can be lowered by conducting the ammonolysis reaction in mixed NH 3 /N 2 gas. The obtained BaTaO 2 N samples crystallize in the cubic CaTiO 3 structure type and form 200–300 nm faceted nanocrystals, based on X-ray diffraction, scanning electron microscopy, and HRTEM. Electron paramagnetic resonance spectra observe the Ta( IV ) defects at g = 1.999 and confirm an 11-fold reduction for the product synthesized in mixed (0.13 : 1.0 vol) NH 3 /N 2 gas, equivalent to 1.14 × 10 16 cm −3 Ta( IV ) ions. This optimized BaTaO 2 N has nearly twice the photocatalytic oxygen evolution activity (AQE of 6.78% at 400 nm) of a reference material made with 1.0 atm ammonia and 78% higher photoelectrochemical water oxidation photocurrent (0.9 mA cm −2 at 1.23 V vs. RHE) under simulated sunlight. According to X-ray photoelectron spectroscopy, remaining Ta( IV ) defects are concentrated in the surface region of the BaTaO 2 N particles, where >50% of all Ta ions are found in the +4 oxidation state. This surface Ta( IV ) population can be directly observed in Vibrating Kelvin Probe Surface Photovoltage Spectra (VK-SPV) via its 1.2–1.4 eV photovoltage onset. Here, it suggests that the surface Ta( IV ) ions contribute empty d-states 0.5–0.7 eV below the BaTaO 2 N conduction band edge. These findings highlight how the energetics and concentrations of Ta( IV ) defects influence the photoelectrochemical water oxidation ability of BaTaO 2 N. Additionally, the work establishes ammonolysis with diluted NH 3 as a new tool to minimize defects in BaTaO 2 N and to raise its solar energy conversion efficiency toward its theoretical limit. Because of its simplicity, the reduced ammonia pressure strategy will likely be applicable to other oxynitrides, which generally suffer from overreduction problems during ammonolysis.

Salmanion, Mahya [University of California, Davis,↗

Ferroelectric Al1 − xBxN thin films integrated on Si

Ferroelectric Al0.93B0.07N thin films are prepared (100) Si substrates. The necessary c-axis out-of-plane orientation to observe macroscopic ferroelectric switching was achieved by implementing an initial Ar/N2 plasma treatment, followed by a thin layer of AlN to initiate the desired texture and a 150 nm W layer. The plasma treatment facilitates crystallinity enhancement of the AlN template layer, allowing for subsequent growth of highly oriented W and Al0.93B0.07N layers. The W layer exhibits random in-plane orientation and exclusive (110) out-of-plane orientation with a rocking curve width of 1.4°. When grown on these W surfaces, 175 nm thick Al0.93B0.07N films exhibit random in-plane orientation and exclusive (001) texture with rocking curve full-width-half-max values of 1.6° and RMS roughness values less than 1 nm. Polarization hysteresis measurements show robust hysteresis with coercive field values of 5.4 MV/cm and remanent polarization values of 136 μC/cm2. XPS depth profile analysis suggests that the plasma treatment converts the existing native oxide to a nitrogen rich oxynitride with approximate composition Si3O0.5N3.67. Cross-sectional TEM reveals that the oxynitride interlayer is amorphous and ∼3.4 nm thick, more than double the native oxide thickness measured by multiwavelength ellipsometry, implying that (oxy)nitride growth continues after conversion of the native oxide. This new family of ferroelectric wurtzites is interesting from an integration perspective given their chemical compatibility with mainstream semiconductors. Developing synthesis routes that promote needed texture while preserving compatible processing windows is an important step toward practical integration.

Physics↗

Platinum group metal-free (PGM-free) integrated tandem junction photoelectrochemical (PEC) water splitting devices (Final Technical Report)

This project involves using solar light, a photoabsorber, and a catalyst to split water and produce hydrogen. The Department of Energy (DOE) targets for integrated photoelectrochemical (PEC) cells emphasize cost (electrode cost < $200/m 2 ), performance (> 15% solar to hydrogen (STH) efficiency), and stability (> 6 months electrode lifetime), but most approaches skew toward one specific metric. This project exploited interfacial integration of platinum group metal-free (PGM-free) catalysts on the high performance and high value tandem solar cells to attain or exceed the DOE benchmark STH energy conversion efficiency and durability. This was investigated in two thrusts by developing two PEC devices for direct comparison: the first is a high-performance monolithic device based on state-of-the-art GaInP 2 /GaAs tandem solar cells developed at National Renewable Energy Laboratory (NREL), and the second is a high-value device based on earth-abundant wide band gap photoabsorber materials including oxynitride and hybrid organic-inorganic perovskite (HOIP) coupled with commercial narrow band gap silicon (Si). Both these devices were paired with electrocatalysts developed for this purpose at Rutgers University by adapting them to thin films on the photoabsorbers. These catalysts are based on the hydrogen evolution reaction (HER) catalysts (Ni 5 P 4 ), oxygen evolution reaction (OER) catalysts (LiCo 2 O 4 ) previously developed at Rutgers for high-efficiency electrolyzers. For using the high-performance tandem solar cell and Ni 5 P 4 catalysts, we achieved a durability of > 200 h at a STH efficiency of > 10%. For using the high-value wide band gap single junction solar cells on n + Si, we achieved 0.73 mA/cm 2 at 1.23V vs reversible hydrogen electrode (RHE) and -15.9 mA/cm 2 at 0V vs RHE using the oxynitride- and HOIP-based photoelectrodes, respectively. After comparing their photocurrent densities, we down selected the HOIP photoabsorber to couple Si for fabrication of a tandem photocathode. However, due to the COVID-19 pandemic, both the laboratories at Rutgers and NREL were locked down for three months and reopened only partially in 2020. The HOIP/Si tandem photocathode using robust nickel phosphide thin film catalysts cannot be developed within the project period, which is expected to achieve a STH efficiency of > 20%. Also, the low-cost LiCo 2 O 4 OER catalysts need to replace the benchmark IrO 2 for evaluation. Finally, techonomic analysis of full high-performance and high-value integrated PEC devices needs to be carried out for comparison in terms of cost.

08 HYDROGEN↗

Realizing Wide-Gamut Human-Centric Display Lighting with K 3 AlP 3 O 9 N:Eu 2+

Abstract Computers, televisions, and smartphones are revolutionized by the invention of InGaN blue light-emitting diode (LED) backlighting. Yet, continual exposure to the intense blue LED emission from these modern displays can cause insomnia and mood disorders. Developing “human-centric” backlighting that uses a violet-emitting LED chip and a trichromatic phosphor mixture to generate color images is one approach that addresses this problem. The challenge is finding a blue-emitting phosphor that possesses a sufficiently small Stokes’ shift to efficiently down-convert violet LED light and produce a narrow blue emission. This work reports a new oxynitride phosphor that meets this demand. K 3 AlP 3 O 9 N:Eu 2+ exhibits an unexpectedly narrow (45 nm, 2206 cm -1 ), thermally robust, and efficient blue photoluminescence upon violet excitation. Computational modeling and temperature-dependent optical property measurements reveal that the narrow emission arises from a rare combination of preferential excitation and site-selective quenching. The resulting chromaticity coordinates of K 3 AlP 3 O 9 N:Eu 2+ lie closer to the vertex of the Rec. 2020 than a blue LED chip and provides access to ≈10% more colors than a commercial tablet when combined with commercial red- and green-emitting phosphors. Alongside the wide gamut, tuning the emission from the violet LED and phosphor blend can reduce blue light emissions to produce next-generation, human-centric displays.

36 MATERIALS SCIENCE↗

Plasma-Assisted Epitaxy of Piezoelectric Sc x Al 1-x N Films on Sapphire for Use in Harsh-Environment Microwave Acoustic Sensors

The Sc x Al 1-x N wurtzite structure has been shown theoretically and experimentally to exhibit significantly higher piezoelectric coupling compared to pure AlN. In this work, a plasma-assisted epitaxial growth method has been used to synthesize epitaxial (0002) Sc x Al 1-x N films on c-sapphire substrates from x = 0.07 to 0.30 by co-evaporating high-purity Sc and Al sources in the presence of a nitrogen plasma generated by an RF plasma source. Epitaxial Sc x Al 1-x N films with highly oriented (0002) grains and in-plane registry were produced on c-sapphire substrates that were pre-exposed to the nitrogen plasma to form an oxynitride seed layer. Growth of Sc x Al 1-x N films was carried out at 930°C under both metal-rich and N-rich conditions using precisely controlled Sc, Al, and N-plasma fluxes. Metal-rich depositions yielded non-(0002)-oriented Sc x Al 1-x N grains and intermetallic ScAl grains. Nitrogen-rich growth with a Sc/Al flux ratio of 1/3 produced the best (0002) epitaxy as determined by x-ray diffraction analysis. Surface acoustic wave resonator (SAWR) devices were fabricated from 500-nm-thick Sc x Al 1-x N and AlN films to extract their electromechanical coupling coefficients, k 2 . As the Sc concentration in the films increases, the degree of (0002) epitaxy is reduced, yet the value of k 2 increases becasue there is more Sc in the wurzite lattice despite the decreased level of (0002) grain alignment. As a result, the use of a 10-nm-thick Si x N y capping layer on top of the Sc x Al 1-x N films aids in preventing etching during SAWR device photolithography and also helps hinder film oxidation up to 800°C.

36 MATERIALS SCIENCE↗

Effect of Thermal Oxidation on the Structure, Surface Texturing, and Microstructure Evolution in Nanocrystalline Ga-O-N Films

An extensive examination of the nanoscale, crystallographic growth dynamics of the system, which is impacted by the thermal energy given to the GaN, is carried out to derive a deeper understanding of the growth kinetics, morphology and microstructure evolution, chemical bonding, and optical properties of Ga-O-N films. Thermal annealing of GaN films is performed in the temperature range of 900–1200 °C. Crystal structure, phase formation, chemical composition, surface morphology, and microstructure evolution of Ga-O-N films are investigated as a function of temperature. Increasing temperature induces surface oxidation, which results in the formation of stable β-Ga2O3 phase in the GaN matrix, where the overall film composition evolves from nitride (GaN) to oxynitride (Ga-O-N). While GaN surfaces are smooth, planar, and featureless, oxidation induced granular-to-rod shaped morphology evolution is seen with increasing temperature to 1200 °C. The considerable texturing and stability of the nanocrystalline Ga-O-N on Si substrates can be attributed to the surface and interface driven modification because of thermal treatment. Corroborating with structure and chemical changes, Raman spectroscopic analyses also indicate that the chemical bonding evolution progresses from fully Ga-N bonds to Ga-O-N. While the GaN oxidation process starts with the formation of β-Ga 2 O 3 at an annealing temperature of 1000 °C, higher annealing temperatures induce structural distortion with the potential formation of Ga-O-N bonds. The structure-phase-chemical composition correlation, which will be useful for nanocrystalline materials for selective optoelectronic applications, is established in Ga-O-N films made by thermal treatment of GaN.

36 MATERIALS SCIENCE↗

Unraveling the Stable Cathode Electrolyte Interface in all Solid‐State Thin‐Film Battery Operating at 5 V

Abstract Spinel‐type LiNi 0.5 Mn 1.5 O 4 (LNMO) is one of the most promising 5 V‐class cathode materials for Li‐ion batteries that can achieve high energy density and low production costs. However, in liquid electrolyte cells, the high voltage causes continuous cell degradation through the oxidative decomposition of carbonate‐based liquid electrolytes. In contrast, some solid‐state electrolytes have a wide electrochemical stability range and can withstand the required oxidative potential. In this work, a thin‐film battery consisting of an LNMO cathode with a solid lithium phosphorus oxynitride (LiPON) electrolyte is tested and their interface before and after cycling is characterized. With Li metal as the anode, this system can deliver stable performance for 600 cycles with an average Coulombic efficiency >99%. Neutron depth profiling indicates a slight overlithiated layer at the interface prior to cycling, a result that is consistent with the excess charge capacity measured during the first cycle. Cryogenic electron microscopy further reveals intimate contact between LNMO and LiPON without noticeable structure and chemical composition evolution after extended cycling, demonstrating the superior stability of LiPON against a high voltage cathode. Consequently, design guidelines are proposed for interface engineering that can accelerate the commercialization of a high voltage cell with solid or liquid electrolytes.

25 ENERGY STORAGE↗

Multiple Functional Bonds Integrated Interphases for Long Cycle Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have garnered significant interest as one of the most promising energy suppliers for power grid energy storage. However, the poor electrode/electrolyte interfacial stability leads to continual electrolyte decomposition and transition metal dissolution, resulting in rapid performance degradation of SIBs. In this work, we propose a strategy integrating multiple functional bonds to regulate electrode/electrolyte interphase by triple-coupling of succinonitrile (SN), sodium hexafluorophosphate (NaPF 6 ) and fluorinated ethylene carbonate (FEC). Theoretical calculation and experiment results show that the solvation structure of Na + and ClO 4 – is effectively reconfigured by the solvated FEC, SN and PF 6 – in PC-based carbonate electrolyte. The newly developed electrolyte demonstrates increased Na + -FEC coordination, weakened interaction of Na + -PC and participation of SN and PF 6 – anions in solvation, resulting in the formation of a conformal interfacial layer comprising of sodium oxynitrides (NaN x O y ), sodium fluoride (NaF) and phosphorus oxide compounds (NaP x O y ). Consequently, a 3 Ah pouch full cell of hard carbon//NaNi 1/3 Fe 1/3 Mn 1/3 O 2 exhibits an excellent capacity retention of 90.4 % after 1000 cycles. Detailed postmortem analysis of interface chemistry is further illustrated by multiple characterization methods. Finally, this study provides a new avenue for developing electrolyte formulations with multiple functional bonds integrated interphases to significantly improve the long-term cycling stability of SIBs.

25 ENERGY STORAGE↗

Ion‐Conducting, Electron‐Blocking Layer for High‐Performance Solid Electrolytes

Lithium metal batteries bring greater promise for energy density, often relying on solid‐state electrolytes to meet critical benchmarks. However, Li dendrite formation is a prevailing problem that limits the cycle life and Coulombic efficiency of solid‐state Li metal batteries. For the first time, a thin (<100 nm) layer of electronically insulating, ionically conducting lithium phosphorus oxynitride (LiPON) is applied using atomic layer deposition between a Li anode and garnet Li 7 La 3 Zr 2 O 12 (LLZO). The performance of a conformal LiPON layer as an electron barrier in symmetric Li‐LLZO cells is observed through potential step chronoamperometry, galvanostatic cycling, electron microscopy, and various spectroscopic techniques. The LiPON‐coated LLZO achieves 100 times lower electronic conductance than LLZO alone. Cycling carried out at 0.1 mA cm −2 for 100 cycles demonstrates that suppression of electron pathways into the bulk solid electrolyte improves the cycle life of a lithium metal cell. These findings suggest an electronic conductivity effect in solid‐state electrolytes. A strategy is demonstrated to design thin‐film (LiPON)‐modulated bulk solid‐state electrolytes (LLZO) capable of maintaining high ionic conductivity and electrochemical stability while reducing the effective electronic conductivity, which results in significantly decreased dendrite formation, improved cycle life, and greater interfacial integrity between the electrolyte and a Li anode.

Hitz, Emily M.↗

Other Power Reactor Fuels

While oxide fuels have received the bulk of historic interest as power reactor fuels, a range of other uranium compounds and deployment architectures have also been considered and demonstrated to varying degrees. Metal alloys, technical ceramics, and oxycarbide or oxynitride particle fuels are candidates for various power reactor applications. Although in general these families of nuclear fuels have received more limited study than oxides, the benefits of uranium density, favorable thermophysical properties, and other factors have resulted in interest from both the traditional nuclear power industry as well as more recent entrepreneurial efforts. The dominant fuel chemistry issues for these fuel forms differ substantially from those of oxide systems. Synthesis routes, factors that affect fresh fuel properties, and critical aspects of fuel performance are unique to these systems and often dominated by the desired chemical behavior of uranium in each system. This chapter will briefly introduce metallic uranium alloys, specifically uranium-zirconium, non-traditional ceramic fuels such as carbides and nitrides, and oxycarbide/oxynitride particle fuels most familiar as the basis for TRISO fuel designs. A survey of major fuel chemistry issues is provided for each of these families of fuels, with a focus on contemporary challenges and active research avenues.

Nelson, Andrew↗

Interaction mechanism of transition metal phthalocyanines on transition metal nitride supports

Here, we investigated the electronic interactions between transition metal phthalocyanine (TMPc’s) on a refractory transition metal nitride support, specifically copper phthalocyanine (CuPc) on titanium nitride (TiN). X-ray Photoelectron Spectroscopy (XPS) results suggest a presence of a few nanometer native oxide layer on the surface of the TiN nanoparticles, which consists of TiN, TiO 2 , and Titanium oxynitrides (Ti x O y N z ). A TiNCuPc nanocomposite was synthesized via a simple mixing method due to the strong binding between CuPc and TiN confirmed by density functional theory (DFT) calculations. Both XPS data and DFT calculations revealed an electron transfer from TiN substrate to CuPc molecule. The nature of charge transfer is not influenced by the presence of an oxide layer on the surface of TiN. Substantial deviations are however found between photoelectron emission microscopy (PEEM) measured work function for TiN (4.68 eV) and theoretically calculated work function for pristine stoichiometric TiN (2.63 eV). This behavior is attributed to the presence of an oxide layer on the TiN surface. TiNCuPc composite system has a work function value between those of TiN and CuPc. Our studies open up an opportunity to apply a new class of materials based on transition metal phthalocyanine/transition metal nitride composites to catalysis and optoelectronic devices.

36 MATERIALS SCIENCE↗

An ultrathin solid-state electrolyte film coated on LiNi 0.8 Co 0.1 Mn 0.1 O 2 electrode surface for enhanced performance of lithium-ion batteries

We report layered Ni-rich oxide is a promising cathode material for lithium-ion batteries (LIBs) of high energy density, yet its poor electrochemical stability induced by electrode-electrolyte interfacial degradation has still needed to be addressed. Surface coating is one of the powerful techniques to tackle this issue, nevertheless, it has been extensively used on particle but not electrode regarding a larger area of protection. Herein, we have covered a nano-scaled layer of lithium phosphorus oxynitride (LiPON) on the electrode level of LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM) surface and found lower impedance, longer cycle life, and better safety for treated battery. Most notably, such an NCM-LiPON based 1.3 Ah pouch cell delivers an energy density of 364.4 Wh kg -1 (based on positive and negative materials), along with the retention of 80.0% over 745 cycles at 0.5 C-rate, which is 1.3 times longer than the bare one. This results from modification of both superficial cathode-electrolyte interphase (CEI) and structure stabilization of general particle surface, therefore, role of conformal coating upon electrode surface should be highlighted.

36 MATERIALS SCIENCE↗

Ultra-thin on-chip ALD LiPON capacitors for high frequency application

Multi-layer ceramic capacitors have been used for high frequency decoupling application due to a lower overall impedance leading to fast current response. However, high parasitic inductance limits the application of these capacitors in ultra-high frequency domain. Thus, Multlayer Ceramic Capacitors (MLCCs) are placed close to the IC to improve circuit efficiency and reduce inductance. With next generation applications, the demand for frequency range has further increased which not only requires enhanced capacitor material but improved manufacturing techniques to limit the inductive path. Here, we demonstrate ALD of two different polymorphs of ultra-thin film lithium phosphorus oxynitride (LiPON) as an inorganic solid state electrolyte (SSE) for on chip capacitors for decoupling application. Both the LiPON capacitors shows an electric double layer behavior with a capacitance of 15 μF/cm 2 and a low leakage current (<20 nA/cm 2 ) at 2V. The LiPON shows EDLC behavior up to 10 kHz and beyond, both the polymorphs show an electrostatic behavior with a high dielectric constant (14). Furthermore, this dual frequency behavior along with low parasitic inductance and on chip integration allows for operation in extended frequency ranges.

25 ENERGY STORAGE↗