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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Diazo coupling for surface attachment of small molecules to TiO 2 nanoparticles

Robust surface attachment of molecular species to metal oxide semiconductors is desirable for many applications. In this work, we report the interfacial diazo coupling of surface-bound amines with aromatics to bind them to the surface of TiO 2 nanoparticles via a robust siloxane anchor and a diazo link. The technique shows potential for the inexpensive, stable, modular and tunable attachment of molecules to metal oxide surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Ion beam-induced bending of TiO 2 nanowires with bead-like and prismatic shapes

Ion beam irradiation is a promising method to manipulate the composition and shape of nanowires. It causes the formation of crystal defects like vacancies and dislocations, and consequently, a volume expansion within the irradiated region, giving rise to the nanowire bending. The bending effect has been extensively discussed within nanowires with different diameters under ion beams with varying energies and ion fluences. However, the behaviors of nanowires with complicated shapes, which may have non-uniform irradiated regions due to the changing angle of incidence and shadowing effect, have remained largely unknown. Herein, the structural changes and bending of TiO 2 nanowires with both bead-like and prismatic shapes are investigated under a Ga + ion beam. The multi-faceted morphology, and consequently, varying angles of incidence, result in inhomogeneous irradiation and volume expansion. As a result, significant bending is only observed in prismatic nanowires. Since irradiation is confined within the half of nanowires facing the ion beam, the bending of nanowires is reversible by changing the direction of the ion beam. In order to provide insights into the tailoring composition and morphology of nanowires, we anticipate that this finding can establish the beam analog at the nanoscale, the bending of which can be tuned by ion irradiation.

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

High performance, electroforming-free, thin film memristors using ionic Na 0.5 Bi 0.5 TiO 3

Here, in ionically conducting Na 0.5 Bi 0.5 TiO 3 (NBT), we explore the link between growth parameters, stoichiometry and resistive switching behavior and show NBT to be a highly tunable system. We show that the combination of oxygen ionic vacancies and low-level electronic conduction is important for controlling Schottky barrier interfacial switching. We achieve a large ON/OFF ratio for high resistance/low resistance ( R HRS / R LRS ), enabled by an almost constant R HRS of ~10 9 Ω, and composition-tunable R LRS value modulated by growth temperature. R HRS / R LRS ratios of up to 10 4 and pronounced resistive switching at low voltages (SET voltage of <1.2 V without high-voltage electroforming), strong endurance (no change in resistance states after several 10 3 cycles), uniformity, stable switching and fast switching speed are achieved. Of particular interest is that the best performance is achieved at the lowest growth temperature studied (600 °C), which is opposite to the case of most other perovskite oxides for memristors, where higher growth temperatures are required for optimum performance. This is understood based on the oxygen vacancy control of interfacial switching in NBT, whereas a range of other mechanisms (including filamentary switching) occur in other perovskites. The study of NBT has enabled us to determine key parameters for achieving high performance memristors.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Temperature dependence of field-response mechanisms in 0.4Ba(Zr 0.2 Ti 0.8 )O 3 &#x2013;0.6(Ba 0.7 Ca 0.3 )TiO 3

Understanding the temperature dependence of how ferroelectric materials respond to electric fields is critical for determining how best to use these materials in applications. In this study, temperature dependent in situ x-ray diffraction is used to study how field-response mechanism Ba ( Zr 0.2 Ti 0.8 ) O 3 – x ( Ba 0.7 Ca 0.3 ) TiO 3 evolves with temperature. Increasing the measurement temperature was found to slightly reduce the extent of domain reversal and the magnitude of the 111 lattice strain. Instead, these results indicate that temperature predominantly impacted the angular dependence of the 002/200 lattice strain. We attribute the observed differences in 002/200 lattice strain to the flattening of the free-energy near the tetragonal-orthorhombic phase boundary, where the flattening of the energy landscape promotes a polarization rotation response during the application of electric fields at room temperature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Er-doped anatase TiO 2 thin films on LaAlO 3 (001) for quantum interconnects (QuICs)

Rare-earth ions (REIs) doped into solid-state crystal hosts offer an attractive platform for realizing quantum interconnects that can function as quantum memories and quantum repeaters. The 4f valence electrons of REIs are shielded by 5s and 5p electrons and undergo highly coherent transitions even when embedded in host crystals. In particular, Er 3+ has an optical transition in the telecom band that is suitable for low-loss communication. Recently, REIs in thin film systems have gained interest due to potential advantages in providing a flexible host crystal environment, enabling scalable on-chip integration with other quantum devices. Here, we investigate the structural and optical properties of Er-doped anatase TiO 2 thin films on LaAlO 3 (001) substrates. By choosing a system with minimal lattice mismatch and adjusting Er-dopant concentration, we achieve optical inhomogeneous linewidths of 5 GHz at 4.5 K. Furthermore, we show that 9 nm-thick buffer and capping layers can reduce the linewidth by more than 40%, suggesting a pathway to further narrowing linewidths in this system. We also identify that Er 3+ ions mainly incorporate into substitutional Ti 4+ sites with non-polar D 2d symmetry, which makes Er dopants insensitive to the first order to local electric fields from impurities and is desirable for coherence properties of Er 3+ spins.

42 ENGINEERING↗
DOE OSTI2023

Elucidating the water–anatase TiO 2 (101) interface structure using infrared signatures and molecular dynamics

The structure and dynamics of water on solid surfaces critically affect the chemistry of materials in ambient and aqueous environments. Here, we investigate the hydrogen bonding network of water adsorbed on the majority (101) surface of anatase TiO 2 , a widely used photocatalyst, using polarization- and azimuth-resolved infrared spectroscopy combined with neural network potential molecular dynamics simulations. Our results show that one monolayer of water saturates the undercoordinated titanium (Ti 5c ) sites, forming one-dimensional chains of molecule hydrogen bonded to surface undercoordinated bridging oxygen (O 2c ) atoms. As the coverage increases, water adsorption on O 2c sites leads to significant restructuring of the water monolayer and the formation of a two-dimensional hydrogen bond network characterized by tightly bound pairs of water molecules on adjacent Ti 5c and O 2c sites. This structural motif likely persists at ambient conditions, influencing the reactions occurring there. In conclusion, the results reported here provide critical details of the structure of the water–anatase (101) interface that were previously hypothesized but unconfirmed experimentally.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Isolation of individual Er quantum emitters in anatase TiO 2 on Si photonics

Defects and dopant atoms in solid state materials are a promising platform for realizing single photon sources and quantum memories, which are the basic building blocks of quantum repeaters needed for long distance quantum networks. In particular, trivalent erbium (Er 3+ ) is of interest because it couples C-band telecom optical transitions with a spin-based memory platform. In order to produce quantum repeaters at the scale required for quantum networks it is imperative to integrate these necessary building blocks with mature and scalable semiconductor processes. Here, in this work, we demonstrate the optical isolation of single Er 3+ ions in CMOS-compatible titanium dioxide (TiO 2 ) thin films monolithically integrated on a silicon-on-insulator photonics platform. Our results demonstrate an initial step toward the realization of a monolithically integrated and scalable quantum photonics package based on Er 3+ doped thin films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Infrared spectral signatures of interfacial water at TiO 2 –electrolyte interfaces from deep potential molecular dynamics

Vibrational spectroscopy is a powerful tool for probing water at oxide–electrolyte interfaces, but its molecular interpretation can be challenging. Here, we employ deep potential long-range molecular dynamics simulations with layer-resolved spectral analysis to investigate the microscopic origins of the infrared (IR) response of water at the interface with anatase TiO 2 (101), a prototypical oxide surface. The calculated interfacial spectra exhibit characteristic modifications compared to bulk water IR spectra, including enhanced intensities, a red shifted and broadened stretching band, and a higher-frequency shoulder, in qualitative agreement with experiments. Spectral decomposition shows that these signatures originate mainly from the first interfacial water layer, dominated by surface-bound H 2 O at Ti 5C sites, with secondary contributions from the second layer. A moderate salt concentration (0.4 M NaCl) leaves both the interfacial structure and the spectra essentially unchanged, while tuning the pH strongly modulates the spectral intensity. We establish a scaling relation linking the spectral intensity to the surface water dissociation fraction and the dipole moment, both governed by interfacial electric fields. These findings provide a microscopic framework for interpreting IR spectra of oxide–electrolyte interfaces.

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

Theoretical and experimental study of temperature effect on electronic and optical properties of TiO 2 : Comparing rutile and anatase

To gain fundamental understanding of the high-temperature optical gas-sensing and light-energy conversion materials, we comparatively investigate the temperature effects on the band gap and optical properties of rutile and anatase TiO 2 experimentally and theoretically. Given that the electronic structures of rutile and anatase are fundamentally different, i.e. direct band gap in rutile and indirect gap in anatase, it is not clear whether these materials exhibit different electronic structure renormalizations with temperature. Using ab initio methods, we show that the electron-phonon interaction is the dominant factor for temperature band gap renormalization compared to the thermal expansion. As a result of different contributions from the acoustic and optical phonons, the band gap is found to widen with temperature up to 300 K, and to narrow at higher temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Incorporation of Ti in epitaxial Fe2TiO4 thin films

The titanomagnetites (Fe2-xTixO4, x = 1) are a family of reducible spinel-structure oxides of interest for their favorable magnetic, catalytic, and electrical transport properties. To understand the stability of the system during low temperature deposition, epitaxial thin films of Fe2TiO4 were deposited by molecular beam epitaxy on MgO(001) at 250 – 375°C. The homogeneous incorporation of Ti, Fe valence, and film morphology were all found to be strongly dependent on the oxidation conditions at the low substrate temperatures employed. More oxidizing conditions led to phase separation into epitaxial, faceted Fe3O4 and rutile TiO2. Less oxidizing conditions resulted in polycrystalline films that exhibited Ti segregation to the film surface, as well as mixed Fe valence (Fe3+, Fe2+, Fe0). A narrow window of intermediate oxygen partial pressure during deposition yielded nearly homogeneous Ti incorporation and a large fraction of Fe2+. However, these films were poorly crystallized, and no occupation of tetrahedral sites in the spinel lattice by Fe2+ was detected by x-ray magnetic circular dichroism at the Fe L-edge. After vacuum annealing, a small fraction of Fe2+ was found to occupy tetrahedral sites. Comparison of these results with previous work suggests that the restriction of the deposition conditions to low temperature by the MgO substrate limits the incorporation of Ti into the spinel lattice. This work suggests a path towards obtaining stoichiometric, well-crystallized Fe2TiO4 by MBE by utilizing high substrate temperature and low oxygen partial pressure during deposition on temperature-stable substrates.

Kaspar, Tiffany C.↗
DOE OSTI2021

Dynamical origins of weakly coupled relaxor behavior in Sn-doped (Ba,Ca)TiO 3 -BiScO 3

The peculiar characteristics of relaxors, viz., a frequency-dependent dielectric permittivity peak and good functional properties (dielectric, electromechanical, electrocaloric, etc.), are attributed to nanoscale regions with correlated dipoles, or polar nanoregions (PNRs). However, the exact nature of PNRs and their contribution to relaxor behavior remains debatable. In recent years, solid solutions of BaTiO3-BiMeO3 (where Me is a metal), have emerged as an interesting system with characteristics in between that of relaxors and dipole glasses. Here, we have examined the atomistic origins of weakly coupled relaxor behavior, specifically with regard to formation of PNRs, in Sn-doped (1-x)(Ba,Ca)TiO 3-x BiScO 3 using macroscopic polarization and neutron dynamic pair distribution function measurements. We show that the short-range atomic correlations observed within the PNRs dynamically fluctuate with frequencies of the order of THz. Furthermore the composition-dependent dielectric and polarization behaviors are critically influenced by the relative stability of the atomic correlations near ~1 THz, while the instantaneous atomic correlations are largely independent of x. The current results are discussed based on a model of intrinsic local modes distributed in a dielectrically soft matrix.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Photoexcitation of bulk polarons in rutile TiO 2

The excitation of surface-localized polaronic states has recently been discussed as an additional photocatalytic channel to band gap excitation for rutile TiO 2 . A contribution from photoexcitation of bulk polarons could, in principle, provide a greater contribution because of their higher number and their protection from oxidation. However, determining such a contribution to the photoyield is challenging and has not been achieved thus far. In this work, we use two photon photoemission spectroscopy measurements to separate bulk and surface polaron photoexcitation. We find that bulk polarons are less bound by 0.2 eV compared with polarons at the surface, consistent with our results of hybrid density functional theory calculations. Because the excited state is also shifted to higher energy, bulk polarons have the same photoexcitation resonance energy as at the surface (3.6 eV) with a threshold at 3.1 eV. This is degenerate with the band gap, suggesting that bulk polarons could also provide an additional contribution to the photoyield.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗