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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 1,567 records · Page 87
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Structural, optical and femtosecond third-order nonlinear optical properties of LiNbO{sub 3} thin films

Highlights: • Fs third order NLO properties of LiNbO{sub 3} thin films were investigated. • Al:ZnO was used as a buffer layer to control the growth of LiNbO{sub 3}. • The magnitude of the χ{sup (3)} of all films was ∼10{sup −11} e.s.u. • χ{sup (3)} obtained here is much higher that of single crystal Z-cut LiNbO{sub 3}. - Abstract: LiNbO{sub 3} thin films were deposited by means of Al:ZnO as intermediate layer using pulsed laser deposition and their structural, linear and nonlinear optical (NLO) properties were investigated. The films were found to be polycrystalline, (110) orientation for Al:ZnO/fused silica and (300) orientation for Al:ZnO/quartz substrates. The films demonstrated a transparency of >80% in the visible and near-infrared region. The NLO studies were performed with the Z-scan technique using ∼150 fs, 80 MHz pulses at 800 nm. The studies revealed that the polycrystalline LiNbO{sub 3}films and (300) LiNbO{sub 3}films demonstrated reverse saturable absorption while a transition to saturable absorption was observed in the case of (110) LiNbO{sub 3}. The presence of two-photon absorption is attributed to the indirect transition via intermediate defect levels which is related to the structural asymmetry. The large nonlinearity in refractive index could be a result of the strain induced ferroelectric polarization. The magnitude of the third order optical nonlinearity of all films was ∼10{sup −11} e.s.u. which is orders of magnitude higher than the value obtained for single crystal Z-cut LiNbO{sub 3}.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Enhanced photocatalytic degradation of methylene blue dye under UV-sunlight irradiation by cesium doped chromium oxide thin films

Highlights: • Cs doped thin films has been prepared by the low- cost spray pyrolysis technique. • Enhanced photocatalytic activity of Cr2O3 thin films for MB photogegradation(MB) dye was achieved through Cs doping. • 6 % Cs doped Cr{sub 2}O{sub 3} thin film photocatalyst with the merits of low band gap and more oxygen vacancies, exhibits the highest sun-light photocatalytic activity. • High photostability and recyclability was achieved under sun-light irradiation. - Abstract: The present contribution highlights the photocatalytic degradation of methylene blue dye over chromium oxide Cr{sub 2}O{sub 3} thin films based photocatalysts illuminated by ultraviolet and sunlight. Cr{sub 2}O{sub 3} photocatalyst was synthesized as thin film by the spray pyrolysis method on glass substrate at temperature of 350 °C. The physical properties of the thin films were studied by X-ray diffraction, scanning electron microscopy (SEM), infrared spectroscopy (IR), Raman spectroscopy, photoluminescence (PL) probing and UV–vis spectroscopy. XRD results showed that all samples were well crystallized with rhomoboedric curudum structure. Emissions of all films have been observed in visible region at 325 nm excitation. UV–vis-NIR spectrophotometry analysis showed that the band gap energy value of chromium oxide is shortened with cesium doping. Results indicate that both the cesium doping and the introduction of oxygen vacancies promote good photocatalytic activity of Cs doped Cr{sub 2}O{sub 3} thin films under sunlight for MB degradation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Integrated carbon cloth supported LiFePO4/N-C films as high-performance cathode for lithium ion batteries

Highlights: • Construct carbon cloth supported LiFePO{sub 4}/N-C films via hydrothermal. • The LiFePO{sub 4}/N-C films show high capacity and good cycles. • Carbon cloth and N-C layer work together to provide fast ion/electron transfer. - Abstract: Fabricating advanced cathode is critical for the development of high-performance lithium ion batteries (LIBs). In this work, we develop a facile hydrothermal method for construction of LiFePO{sub 4}/N-C films on the carbon cloth forming integrated binder-free cathode. The LiFePO{sub 4}/N-C films are composed of interconnected nanoscale LiFePO{sub 4}/N-C particles with diameters of 100–200 nm. High porosity and good adhesion are obtained in the carbon cloth supported LiFePO{sub 4}/N-C films (CC + LiFePO{sub 4}/N-C films). As compared to the LiFePO{sub 4}/C powder nanoparticles, the CC + LiFePO{sub 4}/N-C films show much better electrochemical properties with higher capacities, smaller polarization and better high-rate cycling life. A high capacity of 169 mAh g{sup −1} at 1C is achieved for the carbon cloth supported LiFePO{sub 4}/N-C films, superior to the LiFePO{sub 4}/C powder nanoparticles (154 mAh g{sup −1} at 5C). This enhancement is owing to the unique integrated porous architecture and N-C coating. Our work shows a new way to construct integrated cathode for application in LIBs.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Screen printing large-area organometal halide perovskite thin films for efficient photodetectors

Highlights: • A stable paste with perovskite precursor was produced out. • Screen printing was proposed to fabricate perovskite thin films. • The perovskite photodetectors with symmetric architectures show reasonable performance with high photocurrent and fast response. - Abstract: Organometal halide perovskites have demonstrated excellent features compared to conventional optoelectronic materials. To date, the perovskite films have generally been fabricated by either spin coating or vapor deposition. However, both methods cannot simultaneously meet large scale and low cost of production. Here, a simple, low-cost and high-throughput strategy was proposed to fabricate perovskite thin films, which were further applied in photodetectors. First, lead compound nanoparticles were synthesized by a solvent precipitation method, then a paste with the lead compound nanoparticles was screen printed to form PbO films involving calcination, and next the PbO films directly reacted with CH{sub 3}NH{sub 3}I solution to convert into CH{sub 3}NH{sub 3}PbI{sub 3} films. The CH{sub 3}NH{sub 3}PbI{sub 3} films consisted of micron crystals, whose sizes can be effectively controlled by adjusting CH{sub 3}NH{sub 3}I concentration. Furthermore, the CH{sub 3}NH{sub 3}PbI{sub 3} films were applied in photodetectors, which showed good performance with high sensitivity and fast response.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Al-Si-N thin films deposited by laser ablation: Effect of plasma parameters on mechanical and optical properties

Highlights: • Al-Si-N thin films deposited by PLD showed a chemical composition of the films dependent on the silicon plasma density. • The change in the direct band gap of the samples was related to the Si content and the maximum was 5.54 eV for 20 at.% of Si. • Maximum hardness of 30.3 ± 1.5 GPa was obtained at 4 at.% Si. • The increase in the silicon content of Al-Si-N thin films promotes the formation of a-Si{sub 3}N{sub 4} matrix. - Abstract: Al-Si-N thin films were deposited using simultaneous laser ablation of silicon and aluminum targets in a nitrogen atmosphere at a substrate temperature of 200 °C. The silicon content of the films was studied as a function of the plasma parameters (mean ion kinetic energy and plasma density), produced by the ablation of the silicon target. The plasma parameters were measured by means of a planar Langmuir probe and optical emission spectroscopy. The chemical composition of the films was determined by X-ray photoelectron spectroscopy. The results showed a dependence between the silicon content and the silicon plasma density. Optical emission spectroscopy measurements showed that variations of the working pressure produced changes in the intensity of excited species (N{sub 2}{sup +}, Si{sup 2+}, Al{sup +} and Al{sup 0}), and in the incorporation of these elements into the films. For a working pressure of 0.6 Pa, hardness measurements gave a maximum of 30 ± 1.5 GPa for a silicon content of 4 at.%. The optical constants (refractive index and extinction coefficient) and direct band gap were evaluated as a function of the silicon content in the films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Influence of the substrate temperature on the microstructure and electron-induced secondary electron emission properties of MgO/Au composite film

Highlights: • MgO/Au film was prepared by magnetron sputtering of individual Mg and Au targets. • A higher substrate temperature (T{sub s}) is conducive to the growth of MgO grains. • The surface roughness firstly rises and then decreases with T{sub s} increasing. • The increase of T{sub s} greatly enhances the SEE coefficient of the composite film. - Abstract: The influence of the substrate temperature (T{sub s}) on the microstructure and electron-induced secondary electron emission (SEE) properties of MgO/Au composite film prepared by reactive magnetron sputtering was investigated in order to improve the SEE performance of this composite film. During the deposition of MgO/Au film, a relatively high T{sub s} is conducive to the growth of MgO grains and causes the formation of some large aggregated gold particles on which MgO grains cover in the composite film. In terms of the SEE properties of MgO/Au film, the increase of T{sub s} brings about a significant improvement of SEE coefficient (δ), while it also leads to a slightly accelerated SEE degradation under the continuous electron bombardment due to a faster surface charging. The MgO/Au film prepared at T{sub s} of 773 K has a δ value of 4.9 at a primary electron energy of 200 eV and the maximum δ value of 11.5.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Investigation on microstructures and phase transition characteristics of titanium/yttrium co-doped vanadium oxide thin films

Highlights: • Conjugated interaction between Ti{sup 4+} and Y{sup 3+} ions has been observed in Ti/Y codoped VO{sub 2} films. • The conjugated interaction causes the appearance of VO{sub 2} (B), even complete amorphization in Ti/Y co-doped VO{sub 2} thin film. • Ti/Y co-doping restrains the SMT feature of VO{sub 2} films. - Abstract: Co-doping with two elements has recently attracted attention as a novel approach for tailoring the semiconductor-metal transition (SMT) characteristics of vanadium dioxide (VO{sub 2}). Here, we investigate the influence of titanium/yttrium (Ti/Y) co-doping on the microstructure and SMT characteristics of VO{sub 2} thin films. At low Y concentration, Ti/Y co-doped VO{sub 2} thin films exhibit a monoclinic structure similar to VO{sub 2} but characterized by a smaller grain size, whereas, the VO{sub 2}(B) phase appears as the Y concentration is increased. Interestingly, even a complete amorphization of the film is obtained for co-doped VO{sub 2} thin films. The strong dependence of the microstructure upon the co-doping level is ascribed to the conjugated influence from the simultaneous adjustment of VV chains and VO bonds due to the existence of Ti{sup 4+} and Y{sup 3+} ions in VO{sub 2} lattice, respectively. As a result, the SMT feature of VO{sub 2} thin films is strongly limited or even fully disappears.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Analysis of colorimetry using the CIE-L*a*b* system and the photocatalytic activity of photochromic films

Highlights: • The films containing fibers were efficient in the degradation of methyl orange dye. • Doping of TiO{sub 2} with W allowed the band gap reduction of TiO{sub 2}. • The photochromism observed in the films was due to the synchronicity between oxides. • The films exhibited variations between colors, and different reflectance levels. - Abstract: This work aims to explain how oxide films are correlated when exposed to UV-A radiation, due to the occurrence of similar phenomena. Thus, thin films of TiO{sub 2} and TiO{sub 2} doped with H{sub 2}WO{sub 4} or Na{sub 2}WO{sub 4}·2H{sub 2}O were made by spin-coating, heat treated between 650 and 800 °C and evaluated for their optical and photocatalytic properties. The X-ray diffraction technique (XRD) was used to determine the crystalline structure of the films. The photochromism was observed through a spectrophotometer using the CIE-Lab colorimetric method. The analysis of the films photocatalytic behavior was obtained following the photodegradation in 125 ml of methyl orange dye (20 ppm). The results showed that the films presented good photochromic and photocatalytic properties due to the synchronization between the chemical and physical properties of TiO{sub 2} and tungsten.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Surface morphology and photoelectric properties of FTO ceramic thin films under a simple transparent cover-assisted laser annealing

Highlights: • A simple transparent cover-assisted laser annealing method was employed on FTO films. • The presence of coverslip or PET significantly affected the film surface morphology. • Coverslip- or PET-assisted laser annealing greatly increased grain size of the film. • The PET-assisted laser annealed FTO film exhibited the best photoelectric property. - Abstract: A simple transparent cover-assisted laser annealing strategy, which used a coverglass or a PET sheet as the transparent cover, was proposed to improve photoelectric properties of fluorine-doped tin oxide (FTO) thin films. Pure laser annealing was also performed for comparison. Transparent cover-assisted laser annealing was found to be more effective for producing densely distributed particles, decreasing surface roughness and increasing grain size, thus enhancing transmittance and conductivity of the film. More importantly, compared with coverglass-assisted laser annealing, PET-assisted laser annealing enabled the film surface to be covered with more uniform and denser particles and exhibit a remarkable increase in grain size. Therefore the film had the highest total transmittance of 84.32%, the highest specular transmittance of 75.88%, a slightly decreased haze of 10.01% and the lowest sheet resistance of 7.36 Ω/sq. The simple PET-assisted laser annealing method may be a promising substitution for the conventional laser annealing treatment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

A case study of high-temperature polyetherimide film capacitor fabrication

Capacitors, as the basic building block of power electronics and electrical systems, are the major constraint of the increasingly integrated power systems that require new capable polymer dielectric films operating at higher temperatures >125 °C. Despite tremendous research efforts on the lab scale, there remains a considerable barrier and knowledge basis for converting polymeric films to functioning capacitors desired in actual applications. Here, the authors tackled the polyetherimide (PEI) film scale up issues and developed various engineering processes for film de-wrinkling, optimal metallization, static elimination, and capacitor fabrication improvement. The authors discovered various fabrication challenges such as the electrostatic charge, metallization scheme, winding tension, and end electrode disconnects. To fabricate a high yield of PEI capacitor bobbins, it is necessary to utilize static eliminators (radioactive ionizers), moderately thick aluminum metallization (15–30 Ω/sq), and winding tension (60–100 g). It is also effective to evaluate capacitance, dielectric loss, equivalent series resistance, and thermal cycling stability of capacitors. This work sheds some light on converting PEI films and other polymeric films to capacitors on a scale up fabrication effort. The practical learning from film handling to capacitor fabrication in this work provided the necessary knowledge for manufacturing high-temperature polar film capacitors.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Persistent Conductivity in Post-Growth Doped ZnO Films Following Pulsed UV Laser Irradiation

Solution and rf sputter deposited doped ZnO films were subjected to cumulative 4-nsec pulses of 355 nm light from a pulsed Nd:YAG laser at fluences between 5 and 150 mJ/cm2. Film densification, change in refractive index, and an increase in conductivity were observed following room temperature irradiation in air, a carbon monoxide reducing environment, or under vacuum. At fluences between 20 and 80 mJ/cm2, the films did not damage catastrophically under irradiation and high visible transparency persisted. The increase in conductivity is attributed to creation of oxygen vacancies and subsequent promotion of free carriers into the conduction band. Effects were most pronounced in films treated in vacuum. All treated films became insulating again upon equilibration in air at room temperature after several days. Films were characterized by means of UV-VIS-NIR transmission spectroscopy, Raman spectroscopy and Hall measurements. Analysis of interference fringes in measured transmission spectra allowed evaluation of optical properties. Raman measurements showed an increase of LO mode intensity with respect to TO mode intensity as the films became more conducting in accord with previous work. Results of this study are not only important for continued development of transparent conducting oxides, but also provide compelling evidence for the role of free carriers as initiators of the laser damage process in these wide bandgap metal oxide films.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Effects of particle inclusions on cracking in ultrathin barrier films

Here, the effects of particle inclusions with different mechanical properties on the development of cracks in ultra-thin barrier films have been studied. Two types of particles, alumina and poly(styrene-b-methyl methacrylate) with large differences in their elastic modulus were used, representing both hard and soft inclusions. Particles were seeded onto polyethylene naphthalate substrates and subsequently coated with atomic layer deposited Al 2 O 3 as a barrier film. Ca corrosion tests used to determine the performance of the barrier films revealed that soft polymer particle inclusions severely affect the development of cracking in barrier films as compared to stiff ceramic particles. Tensile tests also confirmed that the barrier films with polymer particle inclusions have the lowest value of critical onset strain. Parametric simulations, conducted by varying the elastic modulus of the particle inclusions over a wide range, resulted in a larger strain energy release rate near particle inclusions which promotes crack formation and negatively impacts the performance of barrier films with softer particle inclusions. Thus, the combination of high residual stresses in the barrier films with the presence of soft particle inclusions is found the most detrimental for the development of high-performance ultrathin barrier films.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Synthesis, Processing, and Use of Isotopically Enriched Epitaxial Oxide Thin Films

Isotopic engineering has emerged as a key approach to study the nucleation, diffusion, phase transitions, and reactions of materials at an atomic level. It aims to uncover mass transport pathways, kinetics, and operational and failure mechanisms of functional materials and devices. Understanding these phenomena leads to deeper insights into important physical processes, such as the transport of ions in energy conversion and storage devices and the role of active sites and supports during heterogeneous catalytic reactions. Likewise, isotopic engineering is being pursued as a means of modifying functionality to enable future technological applications. In this Account, we summarize our recent work employing isotope labeling (e.g., 18 O 2 and 57 Fe) during thin film synthesis and postgrowth processing to reveal growth mechanisms, defect chemistry, and elemental diffusion under working and extreme conditions. Isotope-resolved analysis techniques with nanometer-scale spatial resolution, such as time-of-flight secondary ion mass spectrometry and atom probe tomography, facilitate the accurate quantification of isotopic placement and concentration in our well-defined heterostructures with precisely positioned, isotope-enriched layers. By measuring the nanometer-scale redistribution between natural abundance and isotopically enriched oxygen layers during the deposition of Fe 2 O 3 and Cr 2 O 3 by molecular beam epitaxy, we identified intermixing processes driven by surface adatoms occurring both at the film growth surface and within the first few layers below the surface. Further insights into synthesis mechanisms were gained by studying the tungsten oxide thin films grown by evaporating WO 3 powder in the presence of background 18 O 2 , revealing minimal incorporation of background oxygen during the film formation process. Thermal and radiation-enhanced diffusion in epitaxial Fe and Cr oxides were precisely tracked using 18 O and 57 Fe tracer layers incorporated into model epitaxial oxide thin films. This approach has allowed us to access thermal diffusion behavior at lower temperatures than previously measured, revealing a potential changeover in diffusion mechanism. Understanding radiation-enhanced diffusion in model oxides that represent the surface layers on the structural components of nuclear reactors informs our understanding of their corrosion behavior under irradiation. Isotopic labeling can also provide unique insights into the surface exchange reactions and defect chemistry of electrocatalysts. For instance, tracking the change in 18 O concentration at the surface of an epitaxial LaNiO 3 thin film after the electrocatalytic oxygen evolution reaction revealed the participation of lattice oxygen, confirming a hypothesis that had been proposed previously. Lastly, we highlight a new direction wherein we perform in situ processing studies utilizing isotopic tracers in conjunction with model epitaxial thin films within the atom probe tomography instrument. Additionally, this Account illustrates the great potential of isotopic engineering to enable fundamental mechanistic insights into physical processes and engineer functional properties in epitaxial films, heterostructures, and superlattices.

36 MATERIALS SCIENCE↗
DOE OSTI2021

In Situ Growth and Interlayer Modulation of Layered Double Hydroxide Thin Films from a Transparent Conducting Oxide Precursor

Layered double hydroxides (LDHs) are a class of cationic-layered solids that can be synthetically designed for a variety of advanced functions. Facile thin film growth of LDHs is an important requisite for a variety of applications including functional coatings, displays, and sensing. In this work we demonstrate, for the first time, an in-situ and patternable thin film synthesis of interconnected Zn-Cr LDH particles from a transparent conducting oxide (TCO) precursor, aluminum-doped zinc oxide (AZO), at room temperature within minutes. Synthetic parameters such as chromium (III) nitrate concentration, solvent composition, and reaction time were found to significantly affect the thickness and morphology of the resulting LDH films. These LDH thin films can undergo interlayer anion exchange, which modulates the interlayer distance of the LDH sheets and surface energy of the thin film. Replacement of the interlayer anion with perfluorooctanoate increases the interlayer sheet distance from 0.9 nm to 2.8 nm and induces a super-hydrophobic thin film that is capable of adsolubilizing and retaining organic guest molecules. The synthetic method and structural analysis of the LDH thin films introduced in this work opens new avenues of application for LDH films.

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

Formation of Vertically Oriented Channels during Calcination of Surfactant-Templated Titania-Doped Mesoporous Silica Thin Films

Here, the mesoporous structure development during calcination of titania-doped mesoporous silica thin films is characterized by in situ grazing-incidence small angle x-ray scattering (GISAXS). Varying amounts of titania (≤ 6 mol%) are incorporated in cetyltrimethylammonium bromide (CTAB) templated film by two methods in this study: dispersion of titania in the silica matrix, and fixing titania at the pore surface by complexation of the titania precursor with a sugar-based co-surfactant, dodecyl maltoside (C 12 G 2 ). Before calcination, films templated with only CTAB display a mesoporous structure with a mixture of in-plane 2D hexagonal and randomly oriented porous structure, while C 12 G 2 -complexed film possessed randomly oriented mesostructure only. Both methods yield a final structure of hexagonal close-packed pores orthogonally oriented to the substrate after calcination at 500 °C. This is attributed to weak Si-O-Ti bonds which allow sintering to occur at 500 °C, combined with unidirectional thermal contraction of the film in the vertical direction. Anisotropic stress and annealing of the films allows the randomly oriented pores to merge vertically and form channels with pore diameter of 2.3 ± 0.3 nm. Titania doping greater than 1% is required for this transformation in films with no C 12 G 2 surfactant used while the transformation was observed for films with C 12 G 2 complexation even at low (0.005%) titania doping, perhaps accentuated by orientational disorder introduced by the sugar surfactant.

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

Quantum Size Effects, Multiple Dirac Cones, and Edge States in Ultrathin Bi(110) Films

The presence of inherently strong spin–orbit coupling in bismuth, its unique layer-dependent band topology and high carrier mobility make it an interesting system for both fundamental studies and applications. Theoretically, it has been suggested that strong quantum size effects should be present in the Bi(110) films, with the possibility of Dirac Fermion states in the odd-bilayer (BL) films, originating from dangling p z orbitals and quantum-spin hall (QSH) states in the even-bilayer films. However, the experimental verification of these claims has been lacking. Here, we study the electronic structure of Bi(110) films grown on a high-T c superconductor, Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) using angle-resolved photoemission spectroscopy (ARPES). We observe an oscillatory behavior of electronic structure with the film thickness and identify the Dirac-states in the odd-bilayer films, consistent with the theoretical predictions. In the even-bilayer films, we find another Dirac state that was predicted to play a crucial role in the QSH effect. In the low thickness limit, we observe several extremely one-dimensional states, probably originating from the edge-states of Bi(110) islands. Our results provide a much needed experimental insight into the electronic and structural properties of Bi(110) films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗