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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 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Electrochemically Mediated Au–C(sp 2 ) Anchors for Molecular Electronics

Terminal anchor groups play a key role in the stability and electronic properties of molecular junctions. Single molecule junctions typically consist of two preinstalled terminal anchors linking organic molecules to metal electrodes. Here, in this work, we show that p -terphenyl derivatives containing only a single terminal anchor show conductance features similar to junctions with two preinstalled terminal anchors. A set of p -terphenyl derivatives with one terminal anchor was prepared using automated chemical synthesis and characterized using single molecule electronics experiments, molecular dynamics (MD) simulations, bulk electrochemistry and spectroscopy, and nonequilibrium Green’s function-density functional theory (NEGF-DFT) calculations. Our results show that 4-amino- p -terphenyl (PPP) and related analogs exhibit a well-defined high conductance state that is diminished or absent in other p-terphenyl derivatives lacking a preinstalled amine terminal anchor or fluorine or methyl substitutions at the terminal para position. However, a low conductance state is observed in all amino- p -terphenyl derivatives with one preinstalled anchor due to molecular junctions formed by noncovalent dimeric π–π stacking interactions. The observed high conductance state diminishes upon the addition of reducing agents and is restored upon the addition of an oxidizing agent. Our results suggest that the high conductance state arises due to Au–C(sp 2 ) bond formation facilitated by a single electron oxidation event at the electrode surface. A series of control experiments with different anchor groups shows that primary amines play a key role in forming Au–C bonds for molecular junctions. Overall, these results suggest that Au–C bond formation gives rise to high conductance pathways in organic molecules containing only one preinstalled terminal anchor. Insights from this work can be leveraged in the design of molecular electronic devices, particularly in understanding the mechanisms of molecular binding and junction formation.

charge transport↗
DOE OSTI2023

Andreev reflection between aluminum and graphene across van der Waals barriers

We present planar aluminum superconductor–graphene junctions whose hybrid interface is engineered for couplings ranging from tunneling to the strongly coupled regime by employing an atomically thin van der Waals tunneling barrier. Without the vdW barrier, we find Al makes strongly coupled contacts with the fully proximities graphene channel underneath. Using a large band gap hexagonal boron nitride (hBN) barrier, we find the junctions always remain in the weak coupling regime, exhibiting tunneling characteristics. Using monolayer semi-conducting transition metal dichalcogenides (TMDs) such as MoS2, we realize intermediate coupling with enhanced junction conductance due to the Andreev process. In this intermediate regime, we find that junction resistance changes in discrete steps when sweeping a perpendicular magnetic field. The period of the resistance steps in the magnetic field is inversely proportional to the junction area, suggesting the physical origin of our observations is due to magnetic-field-induced vortex formation in the planar junction.

Physics↗
DOE OSTI2024

Transport signatures of phase fluctuations in superconducting qubits

Abstract Josephson junctions supply the nonlinear inductance element in superconducting qubits. In the widely used transmon configuration, where the junction is shunted by a large capacitor, the low charging energy minimizes the sensitivity of the qubit to charge noise while maintaining the necessary anharmonicity to qubit states. We report here low-frequency transport measurements on small standalone junctions and identically fabricated capacitively-shunted junctions that show two distinct features normally attributed to small capacitance junctions near zero bias: reduced switching currents and prominent finite resistance associated with phase diffusion in the current–voltage characteristic. Our transport data reveals the existence of phase fluctuations in transmons arising from intrinsic junction capacitance.

Wisne, M. (ORCID:0009000610684209)↗
DOE OSTI2025

Anomalous superconductivity in twisted MoTe 2 nanojunctions

Introducing superconductivity in topological materials can lead to innovative electronic phases and device functionalities. Here, we present a unique strategy for quantum engineering of superconducting junctions in moiré materials through direct, on-chip, and fully encapsulated 2D crystal growth. We achieve robust and designable superconductivity in Pd-metalized twisted bilayer molybdenum ditelluride (MoTe 2 ) and observe anomalous superconducting effects in high-quality junctions across ~20 moiré cells. Unexpectedly, the junction develops enhanced, instead of weakened, superconducting behaviors, exhibiting fluctuations to a higher critical magnetic field compared to its adjacent Pd 7 MoTe 2 superconductor. In addition, the critical current further exhibits a notable V-shaped minimum at zero magnetic field. These features are unexpected in conventional Josephson junctions and absent in junctions of natural bilayer MoTe 2 created using the same approach. We discuss implications of these observations, including the possible formation of mixed even- and odd-parity superconductivity at the moiré junctions. Our results also demonstrate a pathway to engineer and investigate superconductivity in fractional Chern insulators.

Science & Technology - Other Topics↗
DOE OSTI2020

Wide Band-Gap Perovskites for Tandem Photovoltaics

This research was aimed at development of a mechanically-stacked, perovskite Multi-Junction Solar Cell in which (1) at least one Junction is made of metal-halide perovskite and (2) the Junctions are manufactured in parallel and then stacked to create the final module. This technology presented significant advanced manufacturing challenges and opportunities in getting to scale, including materials scale-up and the development of novel roll-to-roll processing and encapsulation methods. Research conducted was also applicable to a monolithically-integrated perovskite Multi-Junction Solar Cell, in which the two or more Junctions are integrated and electrically connected. The project explored the following technical opportunities in conjunction with the Cyclotron Road mentored entrepreneurial research experience: Fabrication of inorganic heterojunction layers on both sides of a perovskite semiconductor, and modification of interfaces; developed transparent electrode layers on both sides of a perovskite semiconductor; and developed a prototype and optimized perovskite Junction Solar Cell.

14 SOLAR ENERGY↗
NASA NTRS1978

Determination of bulk diffusion lengths for angle-lapped semiconductor material via the scanning electron microscope: A theoretical analysis

A standard procedure for the determination of the minority carrier diffusion length by means of a scanning electron microscope (SEM) consists in scanning across an angle-lapped surface of a P-N junction and measuring the resultant short circuit current I sub sc as a function of beam position. A detailed analysis of the I sub sc originating from this configuration is presented. It is found that, for a point source excitation, the I sub sc depends very simply on x, the variable distance between the surface and the junction edge. The expression for the I sub sc of a planar junction device is well known. If d, the constant distance between the plane of the surface of the semiconductor and the junction edge in the expression for the I of a planar junction is merely replaced by x, the variable distance of the corresponding angle-lapped junction, an expression results which is correct to within a small fraction of a percent as long as the angle between the surfaces, 2 theta sub 1, is smaller than 10 deg.

Vonroos, O.↗
NASA NTRS1978

A simple theory of back surface field /BSF/ solar cells

A theory of an n-p-p/+/ junction is developed, entirely based on Shockley's depletion layer approximation. Under the further assumption of uniform doping the electrical characteristics of solar cells as a function of all relevant parameters (cell thickness, diffusion lengths, etc.) can quickly be ascertained with a minimum of computer time. Two effects contribute to the superior performance of a BSF cell (n-p-p/+/ junction) as compared to an ordinary solar cell (n-p junction). The sharing of the applied voltage among the two junctions (the n-p and the p-p/+/ junction) decreases the dark current and the reflection of minority carriers by the builtin electron field of the p-p/+/ junction increases the short-circuit current. The theory predicts an increase in the open-circuit voltage (Voc) with a decrease in cell thickness. Although the short-circuit current decreases at the same time, the efficiency of the cell is virtually unaltered in going from a thickness of 200 microns to a thickness of 50 microns. The importance of this fact for space missions where large power-to-weight ratios are required is obvious.

Von Roos, O.↗
NASA NTRS1993

Fabrication of photovoltaic laser energy converterby MBE

A laser-energy converter, fabricated by molecular beam epitaxy (MBE), was developed. This converter is a stack of vertical p-n junctions connected in series by low-resistivity, lattice matched CoSi2 layers to achieve a high conversion efficiency. Special high-temperature electron-beam (e-beam) sources were developed especially for the MBE growth of the junctions and CoSi2 layers. Making use of the small (greater than 1.2 percent) lattice mismatch between CoSi2 and Si layers, high-quality and pinhole-free epilayers were achieved, providing a capability of fabricating all the junctions and connecting layers as a single growth process with one pumpdown. Well-defined multiple p-n junctions connected by CoSi2 layers were accomplished by employing a low growth temperature (greater than 700 C) and a low growth rate (less than 0.5 microns/hour). Producing negligible interdiffusion, the low growth temperature and rate also produced negligible pinholes in the CoSi2 layers. For the first time, a stack of three p-n junctions connected by two 10(exp -5) Ohm-cm CoSi2 layers was achieved, meeting the high conversion efficiency requirement. This process can now be optimized for high growth rate to form a practical converter with 10 p-n junctions in the stack.

Lu, Hamilton↗
NASA NTRS1996

Activation of a Ca(2+)-dependent protein kinase involves intramolecular binding of a calmodulin-like regulatory domain

Ca(2+)-dependent protein kinases (CDPKs) are regulated by a C-terminal calmodulin-like domain (CaM-LD). The CaM-LD is connected to the kinase by a short junction sequence which contains a pseudosubstrate autoinhibitor. To understand how the CaM-LD regulates a CDPK, a recombinant CDPK (isoform CPK-1 from Arabidopsis, accession no. L14771) was made as a fusion protein in Escherichia coli. We show here that a truncated CDPK lacking a CaM-LD (e.g. mutant delta NC-26H) can be activated by exogenous calmodulin or an isolated CaM-LD (Kact approximately 2 microM). We propose that Ca2+ activation of a CDPK normally occurs through intramolecular binding of the CaM-LD to the junction. When the junction and CaM-LD are made as two separate polypeptides, the CaM-LD can bind the junction in a Ca(2+)-dependent fashion with a dissociation constant (KD) of 6 x 10(-6) M, as determined by kinetic binding analyses. When the junction and CaM-LD are tethered in a single polypeptide (e.g. in protein JC-1), their ability to engage in bimolecular binding is suppressed (e.g. the tethered CaM-LD cannot bind a separate junction). A mutation which disrupts the putative CaM-LD binding sequence (e.g. substitution LRV-1444 to DLPG) appears to block intramolecular binding, as indicated by the restored ability of a tethered CaM-LD to engage in bimolecular binding. This mutation, in the context of a full-length enzyme (mutant KJM46H), appears to block Ca2+ activation. Thus, a disruption of intramolecular binding correlates with a disruption of the Ca2+ activation mechanism. CDPKs provide the first example of a member of the calmodulin superfamily where a target binding sequence is located within the same polypeptide.

Non-NASA Center↗
NASA NTRS2004

Silicon Carbide Diodes Characterization at High Temperature and Comparison With Silicon Devices

Commercially available silicon carbide (SiC) Schottky diodes from different manufacturers rated at 200, 300, 600, and 1200 V, were electrically tested and characterized as a function of temperature up to 300 C. Electrical tests included both steady state and dynamic tests. Steady state tests produced forward and reverse I-V characteristic curves. Transient tests evaluated the switching performance of the diodes in either a hard-switched DC to DC buck converter or a half-bridge boost converter. For evaluation and comparison purposes, the same tests were performed with current state-of-the-art ultra fast silicon (Si) pn-junction diodes of similar ratings and also a Si Schottky diode. The comparisons made were forward voltage drop at rated current, reverse current at rated voltage, and turn-off peak reverse recovery current and reverse recovery time. In addition, efficiency measurements were taken for the buck DC to DC converter using both the SiC Schottky diodes and the Si pn-junction diodes at different temperatures and frequencies. The test results showed that at high temperature, the forward voltage drop for SiC Schottky diodes is higher than the forward drop of the ultra fast Si pn-junction diodes. As the temperature increased, the forward voltage drop of the SiC Schottky increased while for the ultra fast Si pn-junction diodes, the forward voltage drop decreased as temperature increased. For the elevated temperature steady state reverse voltage tests, the SiC Schottky diodes showed low leakage current at their rated voltage. Likewise, for the transient tests, the SiC Schottky diodes displayed low reverse recovery currents over the range of temperatures tested. Conversely, the Si pn-junction diodes showed increasing peak reverse current values and reverse recovery times with increasing temperature. Efficiency measurements in the DC to DC buck converter showed the advantage of the SiC Schottky diodes over the ultra fast Si pn-junction diodes, especially at the higher temperatures and higher frequencies.

Lebron-Velilla, Ramon C.↗
NASA NTRS2006

Gap/silicon Tandem Solar Cell with Extended Temperature Range

A two-junction solar cell has a bottom solar cell junction of crystalline silicon, and a top solar cell junction of gallium phosphide. A three (or more) junction solar cell has bottom solar cell junctions of silicon, and a top solar cell junction of gallium phosphide. The resulting solar cells exhibit improved extended temperature operation.

Landis, Geoffrey A.↗
DOE OSTI2023

Multi-core high-temperature, irradiation-resistant thermocouple, and related methods

A multi-core thermocouple includes a plurality of wires, an insulation core surrounding the plurality of wires, a sheath surrounding the insulation core, and a plurality of electrical junctions. The plurality of electrical junctions may include a first electrical junction formed between a first wire of the plurality of wires and the sheath at a first axial mid-section of the multi-core thermocouple, the first electrical junction including a first swaged axial mid-section of the sheath and a second electrical junction formed between a second wire of the plurality of wires and the sheath at a second, different axial mid-section of the multi-core thermocouple, the second electrical junction including a second swaged axial mid-section of the sheath.

Skifton, Richard S.↗
DOE OSTI2025

Stress accommodation in nanoscale dolan bridges designed for superconducting qubits

Josephson junctions are the principal circuit element in numerous superconducting quantum information devices and can be readily integrated into large-scale electronics. However, device integration at the wafer scale necessarily depends on having a reliable, high-fidelity, and high-yield fabrication method for creating Josephson junctions. When creating Al/AlO x based superconducting qubits, the standard Josephson junction fabrication method relies on a sub-micron suspended resist bridge, known as a Dolan bridge, which tends to be particularly fragile and can often times fracture during the resist development process, ultimately resulting in device failure. In this work, we demonstrate a unique Josephson junction lithography mask design that incorporates stress-relief channels. Our simulation results show that the addition of stress-relief channels reduces the lateral stress in the Dolan bridge by more than 70% for all the bridge geometries investigated. In practice, our novel mask design significantly increased the survivability of the bridge during device processing, resulting in 100% yield for over 100 Josephson junctions fabricated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Single-Molecule Conductance through Hybrid Radially and Linearly π-Conjugated Macromolecules Reveals an Unusual Intramolecular π-Interaction

We describe the design, synthesis, and single-molecule junction conductance of π-electron molecules bearing both radial and linear π-conjugation pathways, whereby cycloparaphenylene (CPP) radial cores are π-extended linearly with aryl alkyne substituents as models for previously reported CPP-arylene ethynylene conjugated polymers. Although radially and linearly conjugated molecules have been studied previously in isolation as junction-bridging molecular electronic units, this is the first study to examine molecules where both topologies are operative. Our results reveal that the presence of radial CPP components within the junction-spanning pathway leads to a reduction in the conductance of the backbone compared to model linear phenyl substituents. Through tight-binding and DFT-based calculations, we attribute this conductance change to intramolecular van der Waals (vdW) interactions between the CPP ring and the junction-spanning arylene-ethynylene molecular backbone. These interactions induce changes in the dihedral angles of the backbone, leading to a reduced overlap of π orbitals within the molecular junction.

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

Collective neural network behavior in a dynamically driven disordered system of superconducting loops

Collective properties of complex systems composed of many interacting components such as neurons in our brain can be modeled by artificial networks based on disordered systems. We show that a disordered neural network of superconducting loops with Josephson junctions can exhibit computational properties like categorization and associative memory in the time evolution of its state in response to information from external excitations. Superconducting loops can trap multiples of fluxons in many discrete memory configurations defined by the local free energy minima in the configuration space of all possible states. A memory state can be updated by exciting the Josephson junctions to fire or allow the movement of fluxons through the network as the current through them surpasses their critical current thresholds. Simulations performed with a lumped element circuit model of a 4-loop network show that information written through excitations is translated into stable states of trapped flux and their time evolution. Experimental implementation on a high-Tc superconductor YBCO-based 4-loop network shows dynamically stable flux flow in each pathway characterized by the correlations between junction firing statistics. Neural network behavior is observed as energy barriers separating state categories in simulations in response to multiple excitations, and experimentally as junction responses characterizing different flux flow patterns in the network. The state categories that produce these patterns have different temporal stabilities relative to each other and the excitations. This provides strong evidence for time-dependent (short-to-long-term) memories, that are dependent on the geometrical and junction parameters of the loops, as described with a network model.

Josephson junctions↗
DOE OSTI2020

Demonstrating the GaInP/GaAs Three-Terminal Heterojunction Bipolar Transistor Solar Cell

The three-terminal heterojunction bipolar transistor solar cell (HBTSC) concept enables the realization of a monolithic double-junction device with individual current extraction. We present an HBTSC realized by a heterojunction of GaInP and GaAs. The one-sun open-circuit voltage (V OC ) of the top and bottom junctions are 1.33 V and 0.99 V, respectively, while fill factors (FF) are above 80%. At one-sun illumination, reducing one junction's bias from V OC to maximum power point degrades the performance of the other junction only slightly (<; 0.5% efficiency loss). These results demonstrate the potential of the HBTSC concept to produce high-efficiency independently connected double-junction solar cells.

14 SOLAR ENERGY↗
DOE OSTI2022

Microscopic Visualization of Cell-Cell Adhesion Complexes at Micro and Nanoscale

Considerable progress has been made in our knowledge of the morphological and functional varieties of anchoring junctions. Cell-cell adhesion contacts consist of discrete junctional structures responsible for the mechanical coupling of cytoskeletons and allow the transmission of mechanical signals across the cell collective. The three main adhesion complexes are adherens junctions, tight junctions, and desmosomes. Microscopy has played a fundamental role in understanding these adhesion complexes on different levels in both physiological and pathological conditions. In this review, we discuss the main light and electron microscopy techniques used to unravel the structure and composition of the three cell-cell contacts in epithelial and endothelial cells. It functions as a guide to pick the appropriate imaging technique(s) for the adhesion complexes of interest. We also point out the latest techniques that have emerged. At the end, we discuss the problems investigators encounter during their cell-cell adhesion research using microscopic techniques.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Engineering Surface Architectures for Improved Durability in III–V Photocathodes

GaInP2 has shown promise as the wide bandgap top junction in tandem absorber photoelectrochemical (PEC) water splitting devices. Among previously reported dual-junction PEC devices with a GaInP 2 top cell, those with the highest performance incorporate an AlInP 2 window layer (WL) to reduce surface recombination and a thin GaInP2 capping layer (CL) to protect the WL from corrosion in electrolytes. However, the stability of these III-V systems is limited, and durability continues to be a major challenge broadly in the field of PEC water splitting. This work provides a systematic investigation into the durability of GaInP 2 systems, examining the impacts of the window layer and capping layer among single junction pn-GaInP 2 photocathodes coated with an MoS 2 catalytic and protective layer. The photocathode with both a CL and WL demonstrates the highest PEC performance and longest lifetime, producing a significant current for >125 h. In situ optical imaging and post-test characterization illustrate the progression of macroscopic degradation and chemical state. The surface architecture combining an MoS 2 catalyst, CL, and WL can be translated to dual-junction PEC devices with GaInP 2 or other III-V top junctions to enable more efficient and stable PEC systems.

08 HYDROGEN↗