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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 1,423 records · Page 79
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Magnetic stripe domains of [Pt/Co/Cu]{sub 10} multilayer near spin reorientation transition

The dependence of magnetic anisotropy, magnetic domain patterns and magnetization reversal processes in [Pt/Co(t{sub Co})/Cu]{sub 10} film stack epitaxied on Cu (111) substrate have been studied as a function of the Co layer thickness t{sub Co}, by magneto-optic polar Kerr magnetometry and microscopy. We find the film undergoes spin reorientation transition from out-of-plane to in-plane as t{sub Co} increases. The SRT thickness is verified by Rotating-field Magneto-Optic Kerr effect method. The film exhibits the stripe domain structures at remanence with the width decreasing while t{sub Co} approaches SRT. As demonstrated by the first order reversal curve measurement, the magnetization reversal process encompasses irreversible domain nucleation, domain annihilation at large field and reversible domain switching near remanence.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Calculation of electronic properties of multilayer graphene with Monte Carlo method

In this study, the electronic transport properties of bilayer graphene is investigated by an ensemble Monte Carlo method. The bilayer graphene has a quadratic energy dependence on wave vector near the points known as Dirac points in the reciprocal lattice. For bilayer graphene the scatterings due to acoustic and optic phonons and ionized impurities are taken into account. Velocity-time and steady state velocity-applied field curves are obtained and from the slope of velocity-field curves at low fields, the low field mobility of bilayer graphene is obtained. The dependence of mobility of bilayer graphene on temperature, electron concentration, impurity concentration, acoustic and optic deformation constants is investigated and it is observed that the most important mechanism limiting the mobility is the phonon scattering.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Electromagnetic and thermal properties of three-dimensional printed multilayered nano-carbon/poly(lactic) acid structures

A new type of light-weight material produced by 3D printing consisting of nano-carbon doped polymer layer followed by a dielectric polymer layer is proposed. We performed temperature dependent characterization and measured the electromagnetic (EM) response of the samples in the GHz and THz range. The temperature dependent structural characteristics, crystallization, and melting were observed to be strongly affected by the presence and the number of nano-carbon doped layers in the sandwich structure. The electromagnetic measurements show a great potential of such a type of periodic material for electromagnetic compatibility applications in microwave frequency range. Sandwich structures containing only two nano-carbon layers already become not transparent to the microwaves, giving an electromagnetic interference shielding efficiency at the level of 8–15 dB. A sandwich consisting of one nano-carbon doped and one polymer layer is opaque for THz radiation, because of 80% of absorption. These studies serve as a basis for design and realization of specific optimal geometries of meta-surface type with the 3D printing technique, in order to reach a high level of electromagnetic interference shielding performance for real world EM cloaking and EM ecology applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

High performance and transparent multilayer MoS{sub 2} transistors: Tuning Schottky barrier characteristics

Various strategies and mechanisms have been suggested for investigating a Schottky contact behavior in molybdenum disulfide (MoS{sub 2}) thin-film transistor (TFT), which are still in much debate and controversy. As one of promising breakthrough for transparent electronics with a high device performance, we have realized MoS{sub 2} TFTs with source/drain electrodes consisting of transparent bi-layers of a conducting oxide over a thin film of low work function metal. Intercalation of a low work function metal layer, such as aluminum, between MoS{sub 2} and transparent source/drain electrodes makes it possible to optimize the Schottky contact characteristics, resulting in about 24-fold and 3 orders of magnitude enhancement of the field-effect mobility and on-off current ratio, respectively, as well as transmittance of 87.4 % in the visible wavelength range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Modeling of the photodetector based on the multilayer graphene nanoribbons

Graphene nanoribbon (GNR), which has unique properties and advantages, is a crucial component of nanoelectornic devices, especially in the development of photoelectric detectors. In this work, an infrared photodetector based on the structure of stacked multiple-GNRs, which is separated by a little thick barrier layers (made of tungsten disulfide or related materials) to prevent tunneling current, is proposed and modeled. Operation of photoelectric detector is related to the electron cascaded radiative transition in the adjacent GNRs strengthened by the electrons heated due to the incident light. With a developed model, the working principle is analyzed and the relationships for the photocurrent and dark current as functions of the intensity of the incident radiation are derived. The spectral dependence of the responsivity and detectivity for graphene nanoribbons photodetector (GNRs-PT) with different Fermi energy, band gaps and numbers of GNRs layers are analyzed as well. The results demonstrate that the spectral characteristics depend on the GNRs band gap, which shows a potential on GNRs-PT application in the multi-wavelength systems. In addition, GNRs-PT has a better spectrum property and higher responsivity compared to photodetectors based on In{sub x}Ga{sub x}As in room temperature.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Transparent ambipolar organic thin film transistors based on multilayer transparent source-drain electrodes

A fabrication method for transparent ambipolar organic thin film transistors with transparent Sb{sub 2}O{sub 3}/Ag/Sb{sub 2}O{sub 3} (SAS) source and drain electrodes has been developed. A pentacene/N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic di-imide (PTCDI-C13) bilayer heterojunction is used as the active semiconductor. The electrodes are deposited by room temperature electron beam evaporation. The devices are fabricated without damaging the active layers. The SAS electrodes have high transmittance (82.5%) and low sheet resistance (8 Ω/sq). High performance devices with hole and electron mobilities of 0.3 cm{sup 2}/V s and 0.027 cm{sup 2}/V s, respectively, and average visible range transmittance of 72% were obtained. These transistors have potential for transparent logic integrated circuit applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Synthesis of polycaprolactone-titanium oxide multilayer films by nanosecond laser pulses and electrospinning technique for better implant fabrication

In this study, a combination of electrospinning and laser texturing is introduced as a novel method for increasing the biocompatibility of metal implants. Besides having a rough laser treated surface, the implant benefits from the high porosity and better wettability of an electrospun fibrous structure, which is a more favorable environment for cell proliferation. Titanium samples were patterned using a nanosecond laser beam and were placed as collectors in an electrospinning machine. They were then soaked in simulated body fluid for four weeks. Energy Dispersive X-ray and X-Ray Diffraction results indicate significantly more hydroxyapatite formation on laser treated samples with nanoscale fibers deposited on their surface. This shows that having a laser treated surface underneath the fibrous layer can improve short-term biocompatibility even before degradation of fibers. The thermal conductivity of the electrospun layer, measured using a Hot Disk Transient Plane Source instrument and computer code, was shown to be considerably lower than that of titanium and very close to bone. The presence of this layer can therefore be beneficial in making the implant more compatible to a biological medium. In case of dental implants, it was shown that this layer can act as a thermal barrier while a hot beverage is consumed and it can decrease the temperature rise by about 60%, which avoids any possible damage to newly formed cells during the healing period.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Spin-orbit torques in perpendicularly magnetized Ir22Mn78/Co20Fe60B20/MgO multilayer

The current-induced spin-orbit torques (SOTs) in the perpendicularly magnetized Ir 22 Mn 78 / Co 20 Fe 60 B 20 /MgO structures are investigated. The damping- and field-like torques are characterized using a harmonic technique. The spin Hall angle of Ir 22 Mn 78 is determined to be $θ$ SHE $=+0.057 ± 60.002$. The SOT-driven magnetization switching is also demonstrated with the assistance of an external in-plane field. Furthermore, the magneto-optical Kerr effect imaging experiments show that the magnetization switching is realized through domain nucleation and domain wall motion. These results may promise potential practical applications in high-performance SOT devices based on the antiferromagnetic materials.

42 ENGINEERING↗
DOE OSTI2017

Increased mobility and on/off ratio in organic field-effect transistors using low-cost guanine-pentacene multilayers

Layer-by-layer deposited guanine and pentacene in organic field-effect transistors (OFETs) is introduced. Through adjusting the layer thickness ratio of guanine and pentacene, the tradeoff of two electronic parameters in OFETs, charge carrier mobility and current on/off ratio, was controlled. The charge mobility was enhanced by depositing pentacene over and between guanine layers and by increasing the proportion of pentacene in the layer-by-layer system, while the current on/off ratio was increased via the decreased off current induced by the guanine layers. The tunable device performance was mainly ascribed to the trap and dopant neutralizing properties of the guanine layers, which would decrease the density of free hydroxyl groups in the OFETs. Furthermore, the cost of the devices could be reduced remarkably via the adoption of low-cost guanine.

42 ENGINEERING↗
DOE OSTI2018

Characterization and calibration of a multilayer coated Wolter optic for an imager on the Z-machine at Sandia National Laboratories

The need for a time-resolved monochromatic x-ray imaging diagnostic at photon energies >15 keV has motivated the development of a Wolter optic to study x-ray sources on the Z-machine at Sandia National Laboratories. The work is performed in both the LLNL’s x-ray calibration facility and SNL’s micro-focus x-ray lab. Characterizations and calibrations include alignment, measurement of throughput within the field of view (FOV), the point-spread function within the FOV both in and out of focus, and bandpass in the FOV. These results are compared with ray tracing models, showing reasonable agreement.

47 OTHER INSTRUMENTATION↗
DOE OSTI2019

Hard x-ray standing-wave photoemission insights into the structure of an epitaxial Fe/MgO multilayer magnetic tunnel junction

The Fe/MgO magnetic tunnel junction is a classic spintronic system, with current importance technologically and interest for future innovation. The key magnetic properties are linked directly to the structure of hard-to-access buried interfaces, and the Fe and MgO components near the surface are unstable when exposed to air, making a deeper probing, nondestructive, in-situ measurement ideal for this system. We have thus applied hard X-ray photoemission spectroscopy (HXPS) and standing-wave (SW) HXPS in the few kilo-electron-volt energy range to probe the structure of an epitaxially grown MgO/Fe superlattice. The superlattice consists of 9 repeats of MgO grown on Fe by magnetron sputtering on an MgO(001) substrate, with a protective Al 2 O 3 capping layer. We determine through SW-HXPS that 8 of the 9 repeats are similar and ordered, with a period of 33 ± 4 Å, with the minor presence of FeO at the interfaces and a significantly distorted top bilayer with ca. 3 times the oxidation of the lower layers at the top MgO/Fe interface. There is evidence of asymmetrical oxidation on the top and bottom of the Fe layers. We find agreement with dark-field scanning transmission electron microscope (STEM) and X-ray reflectivity measurements. Through the STEM measurements, we confirm an overall epitaxial stack with dislocations and warping at the interfaces of ca. 5 Å. We also note a distinct difference in the top bilayer, especially MgO, with possible Fe inclusions. Finally, we thus demonstrate that SW-HXPS can be used to probe deep buried interfaces of novel magnetic devices with few-angstrom precision.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Probing energy landscapes in multilayer heterostructures: Challenges and opportunities

Quantitative characterization of energy landscapes at buried interfaces is essential for assessing their functionality. This perspective highlights recent developments in reconstructing internal potential profiles based on hard x-ray photoemission experiments that have proved to be a rich source of information. We show that band-edge profiles can be reconstructed from core-level photoelectron spectra by performing a comprehensive search for the best-fit set of associated layer-resolved spectra. The use of hard x-rays allows heterostructures to be probed over length scales comparable to relevant electronic screening lengths.Significantly,this information takes our understanding of such systems to a new level that is not currently achievable using any other experimental technique.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Experimental and modeling study of 1/ f noise in multilayer MoS2 and MoSe2 field-effect transistors

In field-effect transistors (FETs) with two-dimensional (2D) transition metal dichalcogenide channels, the dependence of field-effect mobility on atomic layer thickness has been studied and interpreted in terms of interface scattering and interlayer coupling resistance (Rint). However, a model for 1/f noise, such as in MoS2 and in MoSe2 FETs, for various contact metals and layer number thicknesses has not been reported. In this work, we have experimentally studied current–voltage and 1/f noise on MoS2 and MoSe2 FETs with source and drain contacts of high and low work function metals to understand both the mobility and the noise behavior. We have developed a noise model incorporating layer number dependent Hooge parameters and Rint. The noise and mobility models utilize screening lengths for charge, mobility, and Hooge parameter to describe the variation of these quantities with a layer number. Using our single model topology with appropriate fitting parameters for each material and each contact metal, the model captures the experimentally observed layer thickness dependence of the Hooge parameter. Our noise analysis is fully comprehensive and, hence, could be applied to any 2D layered systems.

Kwon, Jiseok↗
DOE OSTI2022

Thermal conductivity of multilayer polymer-nanocomposite thin films

The development of electrical insulators that are thermally conducting is critical for thermal management applications in many advanced electronics and electrical devices. Here, we synthesized polymer nanocomposite (PNC) films composed of polymers [polyethylenimine, poly(vinylamine), poly(acrylic acid), and poly(ethylene oxide)] and dielectric fillers (montmorillonite clay and hexagonal boron nitride) by layer-by-layer technique. The cross-plane thermal conductivity (k⊥) of the film was measured by the 3ω method. The effect of various factors such as film growth, filler type, filler volume fraction, polymer chemical structures, and temperature on the thermal conductivity is reported. The k⊥ of PNCs with thickness from 37 nm to 1.34 μm was found to be in the range of 0.11 to 0.21 ± 0.02 W m−1 K−1. The k⊥ values were found to be lower than the constituent polymer matrix. The experimental result is compared with existing theoretical models of nanocomposite systems to get insight into heat transfer behavior in such layered films composed of dielectrics and polymers.

Physics↗