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At least 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Accelerated Degradation in a Quasi-Single-Crystalline Layered Oxide Cathode for Lithium-Ion Batteries Caused by Residual Grain Boundaries

The rapidly growing demand of electrical vehicle (EV) requires high-energy-density lithium-ion batteries (LIBs) with excellent cycling stability and safety performance. However, conventional polycrystalline high-Ni cathodes severely suffer from intrinsic chemo-mechanical degradation and fast capacity fade. The emerging single-crystallization strategy offers a promising pathway to improve the chemo-mechanical stability, however, the single-crystallinity of the cathode is not always guaranteed and residual grain boundaries (GBs) could persist in nonideal synthesis conditions, leading to the formation of ‘quasi’ single-crystalline (QSC) cathodes. So far, there is a lack of understanding of the influence of these residual GBs on the electrochemical performance and structural stability. Herein, we investigate the degradation pathway of a QSC high-Ni cathode through transmission electron microscopy and X-ray techniques. The residual GBs caused by insufficient calcination time, dramatically exacerbate the cathode’s chemo-mechanical instability and cycling performance. Our work offers important guidance for the next-generation cathodes for long-life LIBs.

25 ENERGY STORAGE↗
DOE OSTI2022

Exsolution-Driven Surface Transformation in the Host Oxide

Exsolution synthesizes self-assembled metal nanoparticle catalysts via phase precipitation. An overlooked aspect in this method thus far is how exsolution affects the host oxide surface chemistry and structure. Such information is critical as the oxide itself can also contribute to the overall catalytic activity. Here, combining X-ray and electron probes, we investigated the surface transformation of thin-film SrTi 0.65 Fe 0.35 O 3 during Fe 0 exsolution. We found that exsolution generates a highly Fe-deficient near-surface layer of about 2 nm thick. Moreover, the originally single-crystalline oxide near-surface region became partially polycrystalline after exsolution. Such drastic transformations at the surface of the oxide are important because the exsolution-induced nonstoichiometry and grain boundaries can alter the oxide ion transport and oxygen exchange kinetics and, hence, the catalytic activity toward water splitting or hydrogen oxidation reactions. These findings highlight the need to consider the exsolved oxide surface, in addition to the metal nanoparticles, in designing the exsolved nanocatalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2022

High-Mobility Hole Transport in Single-Grain PbSe Quantum Dot Superlattice Transistors

Epitaxially-fused superlattices of colloidal quantum dots (QD epi-SLs) may exhibit electronic minibands and high-mobility charge transport, but electrical measurements of epi-SLs have been limited to large-area, polycrystalline samples in which superlattice grain boundaries and intragrain defects suppress/obscure miniband effects. Systematic measurements of charge transport in individual, highly-ordered epi-SL grains would facilitate the study of minibands in QD films. Here, we demonstrate the air-free fabrication of microscale field-effect transistors (μ-FETs) with channels consisting of single PbSe QD epi-SL grains (2–7 μm channel dimensions) and analyze charge transport in these single-grain devices. The eight devices studied show p-channel or ambipolar transport with a hole mobility as high as 3.5 cm 2 V –1 s –1 at 290 K and 6.5 cm 2 V –1 s –1 at 170–220 K, one order of magnitude larger than that of previous QD solids. The mobility peaks at 150–220 K, but device hysteresis at higher temperatures makes the true mobility–temperature curve uncertain and evidence for miniband transport inconclusive.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Spectra-Orthogonal Optical Anisotropy in Wafer-Scale Molecular Crystal Monolayers

Controlling the spectral and polarization response of two-dimensional (2D) crystals is vital for developing ultrathin platforms for compact optoelectronic devices. However, independently tuning optical anisotropy and spectral response remains challenging in conventional semiconductors due to the intertwined nature of their lattice and electronic structures. Here, we report spectra-orthogonal optical anisotropy─where polarization anisotropy is tuned independently of spectral response─in wafer-scale, one-atom-thick 2D molecular crystal (2DMC) monolayers synthesized on monolayer transition-metal dichalcogenide (TMD) crystals. Utilizing the concomitant spectral consistency and structural tunability of perylene derivatives, we demonstrate tunable optical polarization anisotropy in 2DMCs with similar spectral profiles, as confirmed by room-temperature scanning tunneling microscopy and cross-polarized reflectance microscopy. Additional angle-dependent analysis of the single-crystal and polycrystalline molecular domains reveals an epitaxial relationship between the 2DMC and TMD. In conclusion, our results establish a scalable, molecule-based 2D crystalline platform for unique and tunable functionalities unattainable in covalent 2D solids.

2D materials↗
DOE OSTI2025

Sn-InAs Nanowire Shadow-Defined Josephson Junctions

Hybrid superconductor–semiconductor platforms are foundational to advancing quantum information technologies, motivating the integration of materials with clean interfaces, robust superconductivity, and scalable architectures. Here, in this work, we report the synthesis and analysis of inclined InAs nanowires, conformally coated with β-Sn shells. These nanowires extend in opposite in-plane directions, forming a self-aligned, criss-cross network. This enables the deterministic formation of nanowire-shadow Josephson junctions through angle-controlled, low-temperature Sn deposition. Structural characterization shows uniform polycrystalline β-Sn shells forming a sharp, diffusion-free interface with InAs. Low-temperature transport measurements reveal a hard induced superconducting gap ≈ 600 μeV, switching currents up to ≈ 500 nA, and parallel magnetic field resilience beyond 1T. These results establish β-Sn/InAs nanowire networks as a promising platform for superconducting qubits, low-noise microwave devices, and the exploration of exotic superconducting phases including triplet pairing and topological superconductivity.

B-Sn↗
DOE OSTI2026

Epitaxial Electrodeposition of Fe with Controlled In-Plane Variants for a Reversible Metal Anode in an Aqueous Electrolyte

The development of reversible metal anodes is a key challenge for advancing aqueous battery technologies, particularly for scalable and safe stationary energy storage applications. Here, in this study, we demonstrate a strategy to realize epitaxial electrodeposition of iron (Fe) on single-crystal copper (Cu) substrates in aqueous electrolytes. We compare the electrodeposition behavior of Fe on polycrystalline and single-crystalline Cu substrates, revealing that the latter enables highly uniform, dense, and crystallographically aligned Fe growth. Comprehensive electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) analysis confirms the formation of Fe with specific out-of-plane and in-plane orientations, including well-defined rotational variants. Our findings highlight that epitaxial electrodeposition of Fe can suppress dendritic growth and significantly enhance Coulombic efficiency during plating/stripping cycles. This approach bridges fundamental crystallography with practical electrochemical performance, providing a pathway toward high-efficiency aqueous batteries utilizing Earth-abundant materials.

aqueous battery↗
DOE OSTI2021

Electrochromic Properties of Perovskite NdNiO 3 Thin Films for Smart Windows

Semiconductors with electrically tunable band gaps are of great interest in controlling transparency to electromagnetic radiation. Thin films of perovskite nickelate NdNiO 3 (NNO), a class of correlated oxides, were deposited on single-crystal (LaAlO 3 (LAO)) and polycrystalline (fluorine-doped tin oxide-coated glass (FTO)) substrates by magnetron sputtering, chemical solution deposition (CSD), and atomic layer deposition (ALD). Their electrochromic behaviors were investigated using a three-electrode setup in basic (KOH solution, pH = 12) electrolyte. During bleaching/coloration process, the proton intercalation/deintercalation and simultaneous electron compensation in the NNO lattice under electrical bias led to crossover of the material between the pristine-conducting phase (Ni 3+ ) and the strongly correlated insulating phase (Ni 2+ ), which serves as the working principle for electrochromic (tunable opacity in the visible range) behavior. Cyclic voltammetry (CV) scans demonstrate that NNO films are electrochemically stable in basic solutions for all three film deposition methods explored here. CV scans at varying rates enabled the extraction of diffusion coefficient of protons in thin film NNO, which is similar to 10 –7 cm 2 s –1 among all films studied. Large light transmittance modulation by bleaching and coloration was observed on films grown on both LAO and FTO substrates, suggesting its potential as an electrochromic material candidate for smart windows and optical shutter applications. Here, porous NNO films obtained by chemical solution deposition tend to demonstrate stronger electrochromic activity than dense films grown by sputtering or ALD.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Ultrathin Ruthenium Films on Graphene Buffered SiO 2 via Quasi Van der Waals Epitaxy

In this study, we demonstrate a quasi van der Waals epitaxy approach to prepare single crystalline Ru ultrathin film on large scale, monolayer graphene. Physical and epitaxial properties of bulk, near surface and surface of ultrathin Ru films were comprehensively studied using various structural, morphological, compositional, and electrical characterization techniques. Here, we confirm that Ru can epitaxially grow on single, monolayer graphene using magnetron sputtering at elevated temperature of 600 °C. The epitaxial Ru films with film thickness ranging from 94.2 nm down to 3.9 nm show the (0001) out-of-plane orientation. The epitaxial relationships between Ru and graphene are out-of-plane Ru(0001) || graphene(0001) and in-plane Ru[$11\bar20$] || graphene[$11\bar20$]. All the Ru films show smooth surfaces with root-mean-square roughness less than 0.8 nm and have negligible oxide layer on the surfaces. The Ru films on graphene demonstrate significantly reduced electrical resistivity comparing to their counterpart grown on bare SiO 2 , which show polycrystalline nature. For 7.1 to 3.9 nm film thicknesses, the resistivity of Ru on graphene shows 38 to 45% resistivity decrease from that of Ru film on bare SiO 2 without graphene. Our observations suggest the existence of the above-classical van der Waals interaction between Ru and graphene. On the other hand, graphene is capable of effectively blocking the inter-diffusion/interaction between Ru and SiO 2 during a 1000 °C annealing process.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Influence of the Ozone Dose Time during Atomic Layer Deposition on the Ferroelectric and Pyroelectric Properties of 45 nm-Thick ZrO 2 Films

Over a decade ago, ferroelectricity was discovered in doped HfO 2 thin films. The HfO 2 -based thin films have attracted much attention due to their remarkable scalability and CMOS compatibility. Other than the HfO 2 -based thin films, the undoped ZrO 2 thin films are understudied despite their commonly reported antiferroelectric behavior. However, being of the same fluorite structure as HfO 2 -based thin films, the undoped ZrO 2 also displayed considerable ferroelectricity as demonstrated in recent studies. Here, 45 nm-thick polycrystalline undoped ZrO 2 films are synthesized using atomic layer deposition with different ozone dose times. The ZrO 2 films are crystallized after atomic layer deposition at 350 °C without anneals. In general, the longer ozone dose time causes a lower in-plane tensile stress and oxygen vacancy content, which help facilitate an irreversible non-polar tetragonal to polar orthorhombic phase transition with electric-field cycling. However, the lower in-plane tensile stress and oxygen vacancy content also stabilize the monoclinic phase so that a long ozone dose time (>17.5 s) reduces the ferroelectric behavior. After wake-up cycles, the ZrO 2 thin film with an ozone dose time of 17.5 s exhibits a remanent polarization of 6 μC·cm –2 and a pyroelectric coefficient of -35 μC·K –1 ·m –2 . Moreover, the wake-up behavior is consistent between the ferroelectric and pyroelectric response. As essential factors in optimizing the growth of fluorite-structure thin films for ferroelectric applications, the in-plane tensile stress and oxygen vacancy content significantly influence the ferroelectric and pyroelectric properties. Additionally, the low thermal budget for processing ferroelectric ZrO 2 thin films is valuable for semiconductor back-end-of-line processes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Sputtered p-Type Cu x Zn 1– x S Back Contact to CdTe Solar Cells

As thin-film cadmium telluride (CdTe) solar cells gain prominence, one particular challenge is optimizing contacts and their interfaces to transfer charge without losses in efficiency. Back contact recombination is still significant and will prevent CdTe solar technology from reaching its full potential in device efficiency, and transparent back contacts have not been developed for bifacial solar technology or multijunction solar cells. To address these challenges, here we investigate sputtered Cu x Zn 1– x S as a p-type semi-transparent back contact material to thin-film polycrystalline CdTe solar cells at Cu concentrations x = 0.30, 0.45, and 0.60. This material is selected for its high hole conductivity (160–2120 S cm –1 ), wide optical band gap (2.25–2.75 eV), and variable ionization potential (approximately 6–7 eV) that can be aligned to that of CdTe. We report that without device optimization, CdTe solar cells with these Cu x Zn 1– x S back contacts perform as well as control cells with standard ZnTe:Cu back contacts. We observe no reduction in external quantum efficiency, low contact barrier heights of approximately 0.3 eV, and carrier lifetimes on par with those of baseline CdTe. These cells are relatively stable over one year in air, with V OC and efficiency of the x = 0.30 cell decreasing by only 1 and 3%, respectively. Using scanning electron microscopy and scanning transmission electron microscopy to investigate the Cu x Zn 1– x S/CdTe interface, we demonstrate that the Cu x Zn 1– x S layer segregates into a bilayer of Cu-Te-S and Zn-Cd-S, and thermodynamic reaction calculations support these findings. Despite its bilayer formation, the back contact still functions well. This investigation explains some of the physical mechanisms governing the device stack, inspires future work to understand interfacial chemistry and charge transfer, and elicits optimization to achieve higher-efficiency CdTe cells.

14 SOLAR ENERGY↗
DOE OSTI2021

Highly Efficient Surface Charge Transfer in Fe 2 TiO 5 Epitaxial Thin Film Photoanodes

Pseudobrookite (Fe 2 TiO 5 ) has attracted significant attention as an emerging photoanode for water oxidation due to reports of enhanced performance in polycrystalline heterostructures with α-Fe 2 O 3 or TiO 2 . However, the specific properties and contribution of Fe 2 TiO 5 remain unknown. Here, we present the first photoelectrochemical characterization of epitaxial thin film Fe 2 TiO 5 , an ideal platform for probing the inherent response of this earth-abundant photoanode for solar water splitting. Moreover, by using an electrolyte containing a hole scavenger, we find highly efficient charge transfer at the Fe 2 TiO 5 –electrolyte interface. This notable surface property is exploited in the form of an Fe 2 TiO 5 /α-Fe 2 O 3 heterostructure photoanode, for which we observe a photocurrent density increase by 1 order of magnitude and an onset potential improvement by ~300 mV. Establishing these fundamental properties elucidates the nature of reaction mechanisms on Fe 2 TiO 5 and informs the design of highly efficient water photo-oxidation devices by incorporating this material with other photoanodes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Epitaxial Stabilization and Oxygen Evolution Reaction Activity of Metastable Columbite Iridium Oxide

Non-rutile polymorphs of binary iridium oxide such as columbite IrO 2 (α-IrO 2 ) are promising candidates for highly active acid-stable oxygen evolution reaction (OER) catalysts, yet their synthesis has been challenging due to the dominant thermodynamic stability of rutile IrO 2 (R-IrO 2 ). Here, we report the growth of α-IrO 2 via epitaxial thin films using pulsed laser deposition. We observe that, in competition with R-IrO 2 (100), the films can be optimized to be predominantly (100)-oriented α-IrO 2 . Surprisingly, the activity of α-IrO 2 shows a large discrepancy of ~0.2 V in the overpotential compared to its predicted activity, which is resolved via theoretical calculations to be a crystal orientation effect. This demonstrates that the electrocatalytic activity can be significantly varied upon crystal orientation, a parameter that is difficult to control in conventional polycrystalline systems but accessible in epitaxial thin films. In total, this study demonstrates epitaxial thin film growth as a powerful technique, which can overcome large energetic instabilities on the order of ~300 meV to stabilize metastable material structures inaccessible by bulk synthesis. As a result, this provides unique opportunities to effectively identify the atomic structure of active catalysts by combining investigations of metastable materials with theoretical predictions.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Trap-Assisted Dopant Compensation Prevents Shunting in Poly-Si Passivating Interdigitated Back Contact Silicon Solar Cells

Using a trap-assisted compensation model, we explain why polycrystalline Si (poly-Si) passivating contacts are able to achieve low leakage current between the doped fingers of interdigitated back contact (IBC) monocrystalline Si solar cells despite mixing of boron and phosphorus dopants in the isolation region. The fill factor of IBC solar cells is strongly affected by the electrical isolation region between n- and p-type fingers, as this region is critical in minimizing shunting losses. During fabrication of monocrystalline Si solar cells with poly-Si passivating contacts, the intrinsic poly-Si isolation region inevitably gets contaminated with both p- and n-type dopants. Using dopant profiles measured with time-of-flight secondary ion mass spectrometry and scanning probe measurements of the isolation region, we demonstrate that despite the dopant spreading during cell processing, a well-compensated region between the doped fingers exists that prevents shunting. The trap-assisted dopant compensation mechanism significantly widens the compensated region to tens of microns, where the residual dopant densities are below the trap density. This enables a high-resistivity region, resulting in low shunt current. Using one-dimensional (1-D) finite element diode simulations, we identify the design parameters and experimental conditions under which a sufficiently resistive region can form. Furthermore, our measurements of 2-D local resistivity and work function maps across the isolation region using scanning spreading resistance microscopy and Kelvin probe force microscopy demonstrate the existence of a highly resistive, wide compensated region and confirm our proposed compensation mechanism. For our structures, this region is ~25 µm in width within a ~150 µm wide finger isolation region with nearly 3 orders of magnitude higher resistivity than the regions dominated by a single type of dopant.

14 SOLAR ENERGY↗
DOE OSTI2023

Water-Assisted Lift-Off Process for Flexible CdTe Solar Cells

In recent years, various lift-off methods have been developed to fabricate lightweight and flexible cadmium telluride (CdTe)-based solar cells. In this work, we report flexible cadmium sulfide (CdS)/CdTe solar cells using a water-assisted lift-off approach. Here we demonstrate that an additional cadmium chloride treatment for the CdS film is critical to improve the grain size of CdS and the post-deposited CdTe film and the electron conductivity of the CdS film. As a result, the quality of the CdS/CdTe front junction is significantly improved with reduced nonradiative recombination. Finally, our lightweight and flexible polycrystalline CdTe solar cells demonstrate a champion power conversion efficiency of 12.6%.

14 SOLAR ENERGY↗
DOE OSTI2023

Hydrogen Stability and Bonding in SiN x and Al 2 O 3 Dielectric Stacks on Poly-Si/SiO x Passivating Contacts

Polycrystalline Si on SiO x passivating contacts enables some of the highest efficiency single-junction Si photovoltaic devices, but the high-temperature firing process needed for industrial metallization can significantly reduce passivation. We show that after firing, the implied open-circuit voltage, iV oc , for the Al 2 O 3 /SiN x /poly-Si/SiO x /c-Si stack is 20-30 mV higher than the SiNx/Al 2 O 3 /poly-Si/SiO x /c-Si stack and therefore provides better passivation of the SiO x /c-Si interface. Using effusion measurements and Fourier transform infrared spectroscopy, we demonstrate that more than twice as much hydrogen is retained in the dielectric up to the peak firing temperature of ~800 degrees C for Al 2 O 3 -capped structures. If the Al 2 O 3 layer is not present in the stack, after firing, the iVoc is lower by 50-100 mV compared to Al 2 O 3 /SiN x or SiN x /Al 2 O 3 stacks. These studies will inform future work on the role of dielectrics in aiding the passivation of poly-Si/SiO x passivating contacts.

08 HYDROGEN↗
DOE OSTI2024

Unveiling Morphology and Crystallinity Dynamics in Ni x Mn 1– x CO 3 Cathode Precursors through Batch-Mode Coprecipitation

This study delves into the synthesis and control of Ni x Mn 1–x CO 3 , a critical class of Mn-rich, Co-free precursors vital for cathode-oxide materials in energy storage and conversion technologies. Employing batch-mode coprecipitation, we systematically generated samples with varying Ni concentrations (x = 0, 0.1, 0.3, 0.5, 0.7, and 0.9) and conducted a comprehensive analysis of their compositions, crystallinities, transition-metal distributions, and particle morphologies through both experimental and computational methods. A significant variation in particle size and crystallinity was observed, contingent on the Ni content. Further, a pivotal transition emerged at Ni concentrations above x = ~0.5, transforming uniform morphologies, such as spherical, monodisperse, pseudo-single-crystalline particles, into bimodal, polycrystalline structures. Furthermore, the study highlights the role of Ni–ammonia complexes leading to Ni-deficient precipitates and underscores the importance of ammonia concentration in achieving precise Ni content control. This study unveils critical reaction conditions governing Mn-rich precursor properties that are vital for cathode-oxides, emphasizing the need for meticulous synthetic control and offering the potential for practical applications in advanced energy storage and conversion systems.

25 ENERGY STORAGE↗
DOE OSTI2025

Quaternary Cu 2 TSiS 4 (T = Fe, Mn) Anodes for Li-Ion Batteries

Developing high-capacity and fast-charging anode materials is critical for achieving high-performance Li-ion batteries (LIBs). Herein, polycrystalline quaternary transition metal silicon sulfides, Cu 2 TSiS 4 (T = Fe, Mn), were synthesized using a solid-state method and investigated as anode materials in LIBs. Cu 2 FeSiS 4 retains a reversible capacity of 670 mAh g –1 at 200 mA g –1 for 400 cycles, while Cu 2 MnSiS 4 suffers from a fast capacity loss in the initial 50 cycles. More importantly, Cu 2 FeSiS 4 can maintain a reversible capacity of 379 mAh g –1 after 700 cycles at a high current density of 2 A g –1 , demonstrating high cyclic stability and fast-charging capacity. To further understand the structure degradation and phase transformation, we investigated the postcycling electrodes using multiple techniques, including the scanning electron microscope with energy-dispersive X-ray spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy techniques. The results indicated that Cu 2 FeSiS 4 undergoes reversible phase transitions with Li 2 S as a major product component. To further assess the performance for practical applications, Cu 2 FeSiS 4 was coupled with LiFePO 4 to make LiFePO 4 ||Cu 2 FeSiS 4 full cells, which delivered superior electrochemical performance. These results demonstrate great promise for using quaternary transition metal silicon sulfides as anodes to achieve low-cost and sustainable LIBs.

25 ENERGY STORAGE↗
DOE OSTI2025

Single Crystalline Na 0.67 Ni 0.33 Mn 0.67 O 2 Positive Electrode Material via Molten Salt Synthesis for Sodium Ion Batteries

P2-layered Na 0.67 Ni 0.33 Mn 0.67 O 2 (NNMO) has emerged as a promising positive electrode material for sodium ion batteries due to its appealing electrochemical properties. Synthesis of polycrystalline NNMO (PC-NNMO) materials through conventional calcination of solid precursors remains the prevailing method, where heating occurs in a dry environment with air or O 2 . On the other hand, the molten salt method, where precursors are submerged in molten salt medium during calcination, emerged in recent years to be a scalable technique for more controlled crystal growth and uniform morphology in a variety of materials. Here, we utilize the molten salt method to synthesize single crystalline NNMO (SC-NNMO) materials with enhanced electrochemical properties. The SC-NNMO material exhibits an initial specific discharge capacity of 95 mAh g –1 at a 0.1C rate, retaining approximately 88.5% of its capacity after 100 cycles over a wide voltage range of 2.0–4.2 V. Furthermore, SC-NNMO maintains a capacity retention of 83.9% after 300 cycles at a 1C rate compared to 66.6% for PC-NNMO, indicating excellent long-term cycling stability. This stability is further confirmed by the performance of an SC-NNMO//hard carbon full cell, which retains 90.3% of its capacity after 200 cycles at 1C within a voltage window of 1.9–4.1 V. The enhancement in stability of the SC-NNMO sample is attributed to the single crystalline structure suppressing the undesired P2–O2 phase transition at high voltage. This study also presents an easy, efficient, and straightforward molten salt process for SC-NNMO material synthesis, offering valuable insights into the potential application of such methodology for the large-scale, cost-effective production of various sodium-layered transition metal oxide positive electrode materials for SIBs.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗