Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “STM”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Experimental Operation of a Prototype 750C Advanced Chloride Molten Salt Bellows Valve

To achieve DOE 2030 SunShot targets that reduce the cost of liquid-based solar by an additional 40% to 70% beyond 2018 costs, a more reliable, highly manufacturable flow valve, capable of achieving operational temperatures of >700°C is required [1]. This paper investigates the development of an innovative high-temperature chloride molten salt valve, with operation up to 750°C. This valve is intended to be employed within Gen 3 CSP liquid-based thermal energy storage (TES) systems as well as Gen 4 modular salt reactor (MSR) technologies. This work details the general design and flow testing of a bellows-seal flow control valve (FCV). This design includes an integrated closed-loop thermal control system to ensure robust design for freeze-thaw cycles. The self-contained thermal management STM system, is unique in the salt valve industry since it is an integrated solution to provide a consistent, repeatable alternative to typical heat tracing. Additionally, the design includes the employment of a novel heat pipe valve stem to facilitate enhanced passive thermal management into the valve assembly. This valve stem heat pipe is designed to facilitate natural circulation within the bonnet to ensure robust operation, during both nominal and transient thermal operation. The valve body and trim will be designed using SS316H, consistent with Flowserve Corporation’s existing product base and is code qualified but will utilize clad material for materials corrosion, manufacturing cost reduction and compatibility to ensure design flexibility. A test campaign was performed in this investigation utilizing a novel 750°C ternary chloride (20%NaCl/40%MgCl2/40%KCl by mol. wt. %) molten salt flow loop. A discussion about the design and installation of the valves within this test bed is provided for this investigation. Valve test results from this study assessed Cv curves as well as multiple actuator cycles, under varying thermodynamic and operational mode conditions, which would be characteristic within a commercial molten salt facility. The results indicate nominal operation for the baseline design, though improved performance and reliability is expected with the full designed FCV.

Armijo, Kenneth (ORCID:0000000346832147)↗
DOE OSTI2021

Atomic-Scale Visualization of the Cuprate Pair Density Wave State

A recent discovery of the pair density wave (PDW) order creates a stir in understanding an underlying physics in the pseudogap states of the cuprate high temperature superconductors. Recent Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM) measurements on Bi 2 Sr 2 CaCu 2 O 8+δ identified that spatially uniform d-wave superconductivity (DSC), d-symmetry charge density wave (CDW), and the PDW play important roles in an intertwined fashion. In this paper, we discuss how these different electronic degrees of freedom emerges in electronic structures of the cuprate and discuss how they are related to each other.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Scanning tunneling thermometer

Various examples are provided related to scanning tunneling thermometers and scanning tunneling microscopy (STM) techniques. In one example, a method includes simultaneously measuring conductance and thermopower of a nanostructure by toggling between: applying a time modulated voltage to a nanostructure disposed on an interconnect structure, the time modulated voltage applied at a probe tip positioned over the nanostructure, while measuring a resulting current at a contact of the interconnect structure; and applying a time modulated temperature signal to the nanostructure at the probe tip, while measuring current through a calibrated thermoresistor in series with the probe tip. In another example, a device includes an interconnect structure with connections to a first reservoir and a second reservoir; and a scanning tunneling probe in contact with a probe reservoir. Electrical measurements are simultaneously obtained for temperature and voltage applied to a nanostructure between the reservoirs.

Chinivaranahalli Shastry, Abhay Shankar↗
DOE OSTI2022

Scanning tunneling thermometer

Various examples are provided related to scanning tunneling thermometers and scanning tunneling microscopy (STM) techniques. In one example, a method includes simultaneously measuring conductance and thermopower of a nanostructure by toggling between: applying a time modulated voltage to a nanostructure disposed on an interconnect structure, the time modulated voltage applied at a probe tip positioned over the nanostructure, while measuring a resulting current at a contact of the interconnect structure; and applying a time modulated temperature signal to the nanostructure at the probe tip, while measuring current through a calibrated thermoresistor in series with the probe tip. In another example, a device includes an interconnect structure with connections to a first reservoir and a second reservoir; and a scanning tunneling probe in contact with a probe reservoir. Electrical measurements are simultaneously obtained for temperature and voltage applied to a nanostructure between the reservoirs.

Chinivaranahalli Shastry, Abhay Shankar↗
DOE OSTI2022

Atomically precise control of rotational dynamics in charged rare-earth complexes on a metal surface

Abstract Complexes containing rare-earth ions attract great attention for their technological applications ranging from spintronic devices to quantum information science. While charged rare-earth coordination complexes are ubiquitous in solution, they are challenging to form on materials surfaces that would allow investigations for potential solid-state applications. Here we report formation and atomically precise manipulation of rare-earth complexes on a gold surface. Although they are composed of multiple units held together by electrostatic interactions, the entire complex rotates as a single unit when electrical energy is supplied from a scanning tunneling microscope tip. Despite the hexagonal symmetry of the gold surface, a counterion at the side of the complex guides precise three-fold rotations and 100% control of their rotational directions is achieved using a negative electric field from the scanning probe tip. This work demonstrates that counterions can be used to control dynamics of rare-earth complexes on materials surfaces for quantum and nanomechanical applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Growth Dynamics and Electron Reflectivity in Ultrathin Films of Chiral Heptahelicene on Metal (100) Surfaces Studied by Spin-Polarized Low Energy Electron Microscopy

Interaction of electrons with chiral matter gives rise to interesting phenomena such as the chirality-induced spin selectivity. The interdependence of reflectivity of spin-polarized low energy electrons and the absolute handedness of chiral molecules is investigated. First, the growth of homochiral films of helical aromatic hydrocarbons, so-called helicenes, on a Cu(100) surface is studied by means of low energy electron microscopy and scanning tunneling microscopy in ultrahigh vacuum. As soon as the coverage exceeds one monolayer, double-layer nucleation and growth is favored such that depletion in the first layer occurs. Spatially resolved work function measurements show that second-layer patches have a lower work function than first-layer areas. Reflectivity spectra of spin-polarized electrons do not show any asymmetry between homochiral films of the enantiomers. Laterally resolved work function measurements do not confirm work function differences such as those reported earlier for photoelectron studies of chiral peptide films on ferromagnetic substrates.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Understanding how defects and dopant atoms in copper surface oxides affect reactivity

Copper and its oxides are key catalytic materials, on which reactions often occur at the metal/oxide interface. Here, in this work, we directly connect the induction period observed during methanol-driven reduction of thin-film copper oxides to their atomic-scale structural order. Using temperature-programmed desorption (TPD) methanol titrations combined with scanning tunneling microscopy, we show that highly ordered oxide phases – particularly the “29” structure with its low defect density – exhibit long induction periods and initially low reactivity. The induction period, defined as the number of methanol TPD cycles required to reach half of the maximum formaldehyde yield, scales with oxide order and oxygen coverage. Enhanced reactivity of well-ordered oxides emerges only after repeated methanol adsorption/desorption cycles generate oxygen vacancies and new Cu(111)/Cu x O interfacial sites. In contrast, disordered or sub-stoichiometric oxides, which contain more intrinsic defects and interfaces, are active from the first TPD cycle. We further examine how dilute Pt and Rh dopants influence oxide order and reactivity: 1% Pt increases defect density and catalytic activity, while 1% Rh promotes oxide ordering and longer induction periods. These findings demonstrate that dilute alloying provides a potential method for tuning the structure and reactivity of Cu(111)/Cu x O interfaces.

Cu(111)Methanol oxidation↗
DOE OSTI2023

Controlled catalyst-transfer polymerization in graphene nanoribbon synthesis

Exercising direct control over the unusual electronic structures arising from quantum confinement effects in graphene nanoribbons (GNRs) is intimately linked to geometric boundary conditions imposed by the structure of the ribbon. Besides composition and position of substitutional dopant atoms, the symmetry of the unit cell, width, length, and termination of a GNR govern its electronic structure. Here, we present a rational design that integrates each of these interdependent variables within a modular bottom-up synthesis. Our hybrid chemical approach relies on a catalyst-transfer polymerization that establishes excellent control over length, width, and end groups. Complemented by a surface-assisted cyclodehydrogenation step, uniquely enabled by matrix-assisted direct (MAD) transfer protocols, geometry and functional handles encoded in a polymer template are faithfully mapped onto the structure of the corresponding GNR. Bond-resolved scanning tunneling microscopy (BRSTM) and spectroscopy (STS) validate the robust correlation between polymer template design and GNR electronic structure.

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

Thoughts on the past, present and future of UHV surface chemistry and the birth of Single-Atom Alloys

Throughout its relatively short lifetime, ultra-high vacuum (UHV) surface chemistry has progressed quickly. In the 1960's, pioneers like Ertl and Somorjai started the field using single crystals and gained significant insight into catalytic processes by relating surface structure to reactivity. The more recent proliferation of scanning probes has significantly increased the power of the single crystal approach by enabling the atomic-scale structure of active sites to be correlated with their reactivity. In this perspective we briefly discuss how the field developed, identify some challenges, and highlight Single-Atom Alloys (SAAs), a new class of heterogeneous catalyst that was developed from a fundamental surface science approach. However, despite recent successes, funding for fundamental surface science has declined. Academic hires in the discipline are also declining in part due to the start-up costs. We make the case that fundamental UHV surface chemistry is still too young a field to be in recession.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Light- and Chemical-Doping-Induced Magnetic Behavior of Eu Molecular Systems

Variable temperature electron paramagnetic resonance (VT-EPR) was used to investigate the role of the environment and oxidation states of several coordinated Eu compounds. We find that while Eu(III) chelating complexes are diamagnetic, simple chemical reduction results in the formation of paramagnetic species. In agreement with the distorted D 3h symmetry of Eu molecular complexes investigated in this study, the EPR spectrum of reduced complexes showed axially symmetric signals (g ⊥ = 2.001 and g ∥ = 1.994) that were successfully simulated with two Eu isotopes with nuclear spin 5/2 ( 151 Eu and 153 Eu with 48% and 52% natural abundance, respectively) and nuclear g-factors 151 Eu/ 153 Eu = 2.27. Illumination of water-soluble complex Eu(dipic) 3 at 4 K led to the ligand-to-metal charge transfer (LMCT) that resulted in the formation of Eu(II) in a rhombic environment (g x = 2.006, g y = 1.995, g z = 1.988). The existence of LMCT affects the luminescence of Eu(dipic) 3 , and pre-reduction of the complex to Eu(II)(dipic) 3 reversibly reduces red luminescence with the appearance of a weak CT blue luminescence. Furthermore, encapsulation of a large portion of the dipic ligand with Cucurbit[7]uril, a pumpkin-shaped macrocycle, inhibited ligand-to-metal charge transfer, preventing the formation of Eu(II) upon illumination.

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

Observation of Single-Electron Transport and Charging on Individual Point Defects in Atomically Thin WSe 2

Defects significantly impact the properties of 2D semiconductors, providing in-gap quantum states that can serve as a natural platform for single-electron operations, localization sites for excitons to serve as single-photon sources, and potentially quantum spin memories. To date, however, a microscopic observation of such a single-electron transport (SET) behavior has been rarely reported on single defects of 2D semiconductors. In this study, we report a SET and charge-state transition on individual point-defect states buried in the bilayer WSe 2 using scanning tunneling microscopy. SET characteristics of their states is evidenced by both the Coulomb staircase and the saturation behavior of the transport current, consistent with the SET model. Furthermore, we demonstrate that, through the local field of a scanning tunneling microscope tip, it is possible to successively charge the defects by single electrons, suggested by both the ring structures and charging peaks in dI/dV measurements. Our results provide new insights into the quantum nature of these defect-bound states, as well as the possibility of using their single-electron behavior and response for applications in quantum information and local-field sensing.

2D semiconductor↗
DOE OSTI2022

Revealing an Intermediate Cu–O/OH Superstructure on Cu(110)

Identifying the atomic structure and formation mechanism of intermediates during chemical transformations is challenging because of their short-lived nature. With a combination of microscopic and spectroscopic measurements and first-principles calculations, herein we report the formation of a metastable intermediate Cu–O/OH superstructure during the reaction of hydrogen with oxygen-covered Cu(110). This superstructure resembles the parent c(6 × 2)-O phase and can be termed as c(6 × 2)-(4O+2OH) with OH groups occupying the missing Cu sites between isolated Cu atoms. Using atomistic calculations, we elucidate the reaction pathways leading to the c(6 × 2)-(4O+2OH) formation via both molecular and dissociative H 2 adsorption. In this work, the results demonstrate the complex surface dynamics resulting from the parallel reaction pathways and may open up the possibility of directing the reaction dynamics by deliberately manipulating transient surface structure and composition.

36 MATERIALS SCIENCE↗
DOE OSTI2023

The Role of Surface Hydroxyls in the Mobility of Carboxylates on Surfaces: Dynamics of Acetate on Anatase TiO 2 (101)

The dynamics of reactive intermediates are important in catalysis for understanding transient species, which can drive reactivity and the transport of species to reaction centers. In particular, the interplay between surface-bound carboxylic acids and carboxylates is important for numerous chemical transformations, including CO 2 hydrogenation and ketonization. Here, we investigate the dynamics of acetic acid on anatase TiO 2 (101) using scanning tunneling microscopy experiments and density functional theory calculations. We demonstrate the concomitant diffusion of bidentate acetate and a bridging hydroxyl and provide evidence for the transient formation of molecular monodentate acetic acid. The diffusion rate is strongly dependent on the position of hydroxyl and adjacent acetate(s). A facile three-step diffusion process is proposed consisting of acetate and hydroxyl recombination, acetic acid rotation, and acetic acid dissociation. Here this study clearly demonstrates that the dynamics of bidentate acetate could be important in forming monodentate species, which are proposed to drive selective ketonization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Strain-Modified Raman Responses in Monolayer MoS 2 Nanobubbles Resolved at 5 nm

The formation of nanoscale bubbles is an unavoidable consequence during the transfer of two-dimensional materials onto target substrates, driven by van der Waals interactions at the interface. While often viewed as imperfections, these nanoscale bubbles have garnered considerable scientific interest due to the substantial in-plane strain gradients they induce, which in turn give rise to a variety of intriguing optoelectronic effects, particularly in semiconducting transition metal dichalcogenides. Determining and analyzing the strain distribution within nanobubbles at the nanoscale is crucial for advancing our understanding of these underlying strain-induced effects. Here, we present a high-resolution scanning tunneling microscopy-based tip-enhanced Raman spectroscopic investigation of localized nanoscale strain distribution within the nanobubbles formed between monolayer MoS 2 and Au interface. By employing cryogenic temperature (78 K), we successfully differentiate the nanoscale Raman signatures between nanobubble edge and pristine MoS 2 . We verify a maximum tensile strain of ∽1.15–1.34% at the nanobubble edge, which gradually diminishes toward the center, yielding a cross-sectional strain profile consistent with a doughnut-shaped distribution. Furthermore, we report to achieve ∽5 nm spatial resolution in probing such edge-localized strain within the nanobubble. In addition, comparative average strain analysis of such MoS 2 nanobubbles is conducted via geometric mechanistic analysis such as membrane and nonlinear plate theories, providing key insight into the geometric nature near the bubble edge. Our work provide fundamental information about strain-induced nanoscale chemical understanding of 2D materials on the nanometer scale, paving the way for practical applications of nanobubbles in strain-engineered optoelectronic devices.

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

Imaging Quantum Interference in Stadium-Shaped Monolayer and Bilayer Graphene Quantum Dots

Experimental realizations of graphene-based stadium-shaped quantum dots (QDs) have been few and have been incompatible with scanned probe microscopy. Yet, the direct visualization of electronic states within these QDs is crucial for determining the existence of quantum chaos in these systems. Here we report the fabrication and characterization of electrostatically defined stadium-shaped QDs in heterostructure devices composed of monolayer graphene (MLG) and bilayer graphene (BLG). To realize a stadium-shaped QD, we utilized the tip of a scanning tunneling microscope to charge defects in a supporting hexagonal boron nitride flake. The stadium states visualized are consistent with tight-binding-based simulations but lack clear quantum chaos signatures. The absence of quantum chaos features in MLG-based stadium QDs is attributed to the leaky nature of the confinement potential due to Klein tunneling. In contrast, for BLG-based stadium QDs (which have stronger confinement) quantum chaos is precluded by the smooth confinement potential which reduces interference and mixing between states.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Anisotropic Response of Defect Bound States to the Magnetic Field in Epitaxial FeSn Films

Crystal defects, whether intrinsic or engineered, drive many fundamental phenomena and novel functionalities of quantum materials. Here, we report symmetry-breaking phenomena induced by Sn vacancy defects on the surface of epitaxial Kagome antiferromagnetic FeSn films using low-temperature scanning tunneling microscopy and spectroscopy. Near the single Sn vacancy, anisotropic quasiparticle interference patterns are observed in the differential conductance dI/dV maps, breaking the 6-fold rotational symmetry of the Kagome layer. Furthermore, the Sn vacancy defects induce bound states that exhibit anomalous Zeeman shift under an out-of-plane magnetic field, where the energy of the bound states moves linearly toward higher energy independent of the direction of the magnetic field. Under an in-plane magnetic field, the shift of the bound state energy also shows a 2-fold oscillating behavior as a function of the azimuth angle. These findings demonstrate defectenabled new functionalities in Kagome antiferromagnets for potential applications in nanoscale spintronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

Controllable Formation of Threefold-Coordinated Oxygen in Graphene by Low-Energy Ion Implantation

The atomically precise engineering of impurities in graphene and the understanding of their structural and carrier-dependent electronic properties at the nanoscale are crucial for advancing graphene-based nanoelectronics, catalysis, and energy technologies. Here, we demonstrate controllable incorporation of the elusive 3-fold-coordinated O substitutions into graphene using low-energy O + ion implantation under ultrahigh-vacuum conditions. By combining high-resolution scanning tunneling microscopy and spectroscopy (STM/S), bond-resolved noncontact atomic force microscopy techniques, and density functional theory (DFT) calculations, we resolve both the structural and electronic properties of the O-related defects. The STM/S measurements, corroborated by DFT calculations, uncover a characteristic impurity state that is energetically pinned to the Dirac point across different charge-carrier doping regimes. Molecular dynamics simulations further reveal the distribution of implantation-induced configurations and identify the formation of 3-fold-coordinated O dopants. Furthermore, this work provides a viable route to incorporate 3-fold-coordinated O dopants and opens new opportunities for controlled defect engineering in graphene.

3-fold-coordinated oxygen↗