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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 1,477 records · Page 82
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Reflection High-Energy Electron Diffraction Beam-Induced Structural and Property Changes on WO3 Thin Films

Reduction of transition metal oxides can greatly change their physical and chemical properties. Using deposition of WO3 as a case study, we demonstrate that reflection high-energy electron diffraction (RHEED), a surface-sensitive tool widely used to monitor thin-film deposition processes, can significantly affect the cation valence and physical properties of the films through electron-beam induced sample reduction. The RHEED beam is found to increase film smoothness during epitaxial growth of WO3, as well as change the electronic properties of the film through preferential removal of surface oxygen.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2014

Molecular self ordering and charge transport in layer by layer deposited poly (3,3‴-dialkylquarterthiophene) films formed by Langmuir-Schaefer technique

The performance of π-conjugated polymer based electronic devices is directly governed by the molecular morphology of polymer aggregation, the extent to which a molecule is electronically coupled (self ordered and interacted) to neighboring molecules, and orientation. The well electronic coupled and crystalline/ordered polymer films have the potential to enhance the charge transport properties up to a benchmark. However, there is insufficient knowledge about the direct formation of large area, oriented, crystalline, and smooth films. In this study, we have presented Langmuir Schaefer technique to obtain the large area, oriented, crystalline, and smooth film of Poly (3,3‴-dialkylquarterthiophene) (PQT-12) polymer. The effect of self ordering and orientation of PQT-12 polymer on optical, morphological, and charge transport properties has been investigated. The prepared films have been characterized by UV-vis spectroscopy, Raman spectroscopy, transmission electron microscopy (TEM), selected area diffractions pattern (SAED), and atomic force microscopy (AFM) techniques. UV-vis spectra, TEM, SAED, and AFM images of monolayer films reveal the formation of well ordered and electronically coupled polymer domains. Layer by layer deposited films reveal the change in the orientation, which is confirmed by Raman spectra. Electronic properties and layer dependent charge transport properties are investigated using sandwiched structure Al/PQT-12/ITO Schottky configuration with perpendicular to the deposited films. It is observed that the charge transport properties and device electronic parameters (ideality factor and turn on voltage) are significantly changing with increasing the number of PQT-12 layers. Our study also demonstrates the charge transport between polymer crystallites and cause of deviation of ideal behavior of organic Schottky diodes. It may be further explored for improving the performance of other organic and optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Comparative study of the mechanical properties of nanostructured thin films on stretchable substrates

Comparative studies of the mechanical behavior between copper, tungsten, and W/Cu nanocomposite based on copper dispersoïd thin films were performed under in-situ controlled tensile equi-biaxial loadings using both synchrotron X-ray diffraction and digital image correlation techniques. The films first deform elastically with the lattice strain equal to the true strain given by digital image correlation measurements. The Cu single thin film intrinsic elastic limit of 0.27% is determined below the apparent elastic limit of W and W/Cu nanocomposite thin films, 0.30% and 0.49%, respectively. This difference is found to be driven by the existence of as-deposited residual stresses. Above the elastic limit on the lattice strain-true strain curves, we discriminate two different behaviors presumably footprints of plasticity and fracture. The Cu thin film shows a large transition domain (0.60% true strain range) to a plateau with a smooth evolution of the curve which is associated to peak broadening. In contrast, W and W/Cu nanocomposite thin films show a less smooth and reduced transition domain (0.30% true strain range) to a plateau with no peak broadening. These observations indicate that copper thin film shows some ductility while tungsten/copper nanocomposites thin films are brittle. Fracture resistance of W/Cu nanocomposite thin film is improved thanks to the high compressive residual stress and the elimination of the metastable β-W phase.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Controllable spin-charge transport in strained graphene nanoribbon devices

We theoretically investigate the spin-charge transport in two-terminal device of graphene nanoribbons in the presence of a uniform uniaxial strain, spin-orbit coupling, exchange field, and smooth staggered potential. We show that the direction of applied strain can efficiently tune strain-strength induced oscillation of band-gap of armchair graphene nanoribbon (AGNR). It is also found that electronic conductance in both AGNR and zigzag graphene nanoribbon (ZGNR) oscillates with Rashba spin-orbit coupling akin to the Datta-Das field effect transistor. Two distinct strain response regimes of electronic conductance as function of spin-orbit couplings magnitude are found. In the regime of small strain, conductance of ZGNR presents stronger strain dependence along the longitudinal direction of strain. Whereas for high values of strain shows larger effect for the transversal direction. Furthermore, the local density of states shows that depending on the smoothness of the staggered potential, the edge states of AGNR can either emerge or be suppressed. These emerging states can be determined experimentally by either spatially scanning tunneling microscope or by scanning tunneling spectroscopy. Our findings open up new paradigms of manipulation and control of strained graphene based nanostructure for application on novel topological quantum devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Thermal annealing study on P3HT: PCBM based bulk heterojunction organic solar cells using impedance spectroscopy

Recently, Thermal annealing is an important process for bulk heterojunction organic solar cells (BHJ OSCs) to improve the device efficiency and performance of the organic solar cells. Here in, we have examined the changes in the efficiency and morphology of P3HT: PCBM film according to the thermal annealing temperature to find the changes during the annealing process by measuring the optical absorption, atomic force microscope and X-ray diffraction. We also investigated the effect of different annealing process conditions (without, pre- and post-annealing) on the device performance of the inverted bulk heterojunction organic solar cells consist the structure of ITO/ ZnO / P3HT: PCBM / MoO{sub 3}/ Al by measuring AC impedance characteristics. Particularly, the power conversion efficiency (PCE), crystalline nature of the polymer, light absorption and the surface smoothness of P3HT: PCBM films are significantly improved after the annealing process. These results indicated the improvement in terms of PCE, interface smoothness between the P3HT: PCBM and MoO{sub 3} layers of the post annealed device originated from the decrease of series resistance between P3HT: PCBM layer and Al electrodes, which could be due to decrease in the effective life time of charge carriers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

A predictive model of shell morphology in CdSe/CdS core/shell quantum dots

Lattice mismatch in core/shell nanoparticles occurs when the core and shell materials have different lattice parameters. When there is a significant lattice mismatch, a coherent core-shell interface results in substantial lattice strain energy, which can affect the shell morphology. The shell can be of uniform thickness or can be rough, having thin and thick regions. A smooth shell minimizes the surface energy at the expense of increased lattice strain energy and a rough shell does the opposite. A quantitative treatment of the lattice strain energy in determining the shell morphology of CdSe/CdS core/shell nanoparticles is presented here. We use the inhomogeneity in hole tunneling rates through the shell to adsorbed hole acceptors to quantify the extent of shell thickness inhomogeneity. The results can be understood in terms of a model based on elastic continuum calculations, which indicate that the lattice strain energy depends on both core size and shell thickness. The model assumes thermodynamic equilibrium, i.e., that the shell morphology corresponds to a minimum total (lattice strain plus surface) energy. Comparison with the experimental results indicates that CdSe/CdS nanoparticles undergo an abrupt transition from smooth to rough shells when the total lattice strain energy exceeds about 27 eV or the strain energy density exceeds 0.59 eV/nm{sup 2}. We also find that the predictions of this model are not followed for CdSe/CdS nanoparticles when the shell is deposited at very low temperature and therefore equilibrium is not established.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Epitaxial GeSn film formed by solid phase epitaxy and its application to Yb{sub 2}O{sub 3}-gated GeSn metal-oxide-semiconductor capacitors with sub-nm equivalent oxide thickness

Through the technique of solid phase epitaxy (SPE), an epitaxial Ge{sub 0.955}Sn{sub 0.045} film was formed on a Ge substrate by depositing an amorphous GeSn film followed by a rapid thermal annealing at 550 °C. A process that uses a SiO{sub 2} capping layer on the amorphous GeSn film during SPE was proposed and it prevents Sn precipitation from occurring while maintaining a smooth surface due to the reduced surface mobility of Sn atoms. The high-quality epitaxial GeSn film was observed to have single crystal structure, uniform thickness and composition, and tiny surface roughness with root mean square of 0.56 nm. With a SnO{sub x}-free surface, Yb{sub 2}O{sub 3}-gated GeSn metal-oxide-semiconductor (MOS) capacitors with equivalent oxide thickness (EOT) of 0.55 nm were developed. A small amount of traps inside the Yb{sub 2}O{sub 3} was verified by negligible hysteresis in capacitance measurement. Low leakage current of 0.4 A/cm{sup 2} at gate bias of flatband voltage (V{sub FB})-1 V suggests the high quality of the gate dielectric. In addition, the feasibility of using Yb{sub 2}O{sub 3} to well passivate GeSn surface was also evidenced by the small interface trap density (D{sub it}) of 4.02 × 10{sup 11} eV{sup −1} cm{sup −2}, which can be attributed to smooth GeSn surface and Yb{sub 2}O{sub 3} valency passivation. Both leakage current and D{sub it} performance outperform other passivation techniques at sub-nm EOT regime. The proposed epitaxial GeSn film along with Yb{sub 2}O{sub 3} dielectric paves an alternative way to enable high-performance GeSn MOS devices.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Progress in indirect and direct-drive planar experiments on hydrodynamic instabilities at the ablation front

Understanding and mitigating hydrodynamic instabilities and the fuel mix are the key elements for achieving ignition in Inertial Confinement Fusion. Cryogenic indirect-drive implosions on the National Ignition Facility have evidenced that the ablative Rayleigh-Taylor Instability (RTI) is a driver of the hot spot mix. This motivates the switch to a more flexible higher adiabat implosion design [O. A. Hurricane et al., Phys. Plasmas 21, 056313 (2014)]. The shell instability is also the main candidate for performance degradation in low-adiabat direct drive cryogenic implosions [Goncharov et al., Phys. Plasmas 21, 056315 (2014)]. This paper reviews recent results acquired in planar experiments performed on the OMEGA laser facility and devoted to the modeling and mitigation of hydrodynamic instabilities at the ablation front. In application to the indirect-drive scheme, we describe results obtained with a specific ablator composition such as the laminated ablator or a graded-dopant emulator. In application to the direct drive scheme, we discuss experiments devoted to the study of laser imprinted perturbations with special phase plates. The simulations of the Richtmyer-Meshkov phase reversal during the shock transit phase are challenging, and of crucial interest because this phase sets the seed of the RTI growth. Recent works were dedicated to increasing the accuracy of measurements of the phase inversion. We conclude by presenting a novel imprint mitigation mechanism based on the use of underdense foams. The foams induce laser smoothing by parametric instabilities thus reducing the laser imprint on the CH foil.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Two modes of surface roughening during plasma etching of silicon: Role of ionized etch products

Atomic- or nanometer-scale surface roughening has been investigated during Si etching in inductively coupled Cl{sub 2} plasmas, as a function of rf bias power or ion incident energy E{sub i}, by varying feed gas flow rate, wafer stage temperature, and etching time. The experiments revealed two modes of surface roughening which occur depending on E{sub i}: one is the roughening mode at low E{sub i} < 200–300 eV, where the root-mean-square (rms) roughness of etched surfaces increases with increasing E{sub i}, exhibiting an almost linear increase with time during etching (t < 20 min). The other is the smoothing mode at higher E{sub i}, where the rms surface roughness decreases substantially with E{sub i} down to a low level < 0.4 nm, exhibiting a quasi-steady state after some increase at the initial stage (t < 1 min). Correspondingly, two different behaviors depending on E{sub i} were also observed in the etch rate versus √(E{sub i}) curve, and in the evolution of the power spectral density distribution of surfaces. Such changes from the roughening to smoothing modes with increasing E{sub i} were found to correspond to changes in the predominant ion flux from feed gas ions Cl{sub x}{sup +} to ionized etch products SiCl{sub x}{sup +} caused by the increased etch rates at increased E{sub i}, in view of the results of several plasma diagnostics. Possible mechanisms for the formation and evolution of surface roughness during plasma etching are discussed with the help of Monte Carlo simulations of the surface feature evolution and classical molecular dynamics simulations of etch fundamentals, including stochastic roughening and effects of ion reflection and etch inhibitors.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Stable finite element method of eight-band k·p model without spurious solutions and numerical study of interfaces in heterostructures

A Lagrange-Hermite finite element method for the eight-band k·p model is developed. We demonstrate that besides the incompletion of k·p basis functions, the ill representation of first-order derivatives can also bend the conduction band structure down and lead to the highly oscillatory solutions. Our method simultaneously solves these two problems and achieves robust stability and high accuracy in real-space numerical calculation. The more physical asymmetric operator ordering is employed and the connection problem in abrupt interface is resolved by using an approximately abrupt interface. The situation of smooth interface used to explain the discrepancies between experiment and simulation of abrupt interface is also calculated by our method, and the result suggests that the influence of the interface smoothing should be considered in the short period superlattices or quantum structures of the narrow well.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Beyond the random phase approximation: Stimulated Brillouin backscatter for finite laser coherence times

We developed a linear theory of backward stimulated Brillouin scatter (BSBS) of a spatially and temporally random laser beam relevant for laser fusion. Our analysis reveals a new collective regime of BSBS (CBSBS). Its intensity threshold is controlled by diffraction, once cTc exceeds a laser speckle length, with Tc the laser coherence time. The BSBS spatial gain rate is approximately the sum of that due to CBSBS, and a part which is independent of diffraction and varies linearly with Tc. The CBSBS spatial gain rate may be reduced significantly by the temporal bandwidth of KrF-based laser systems compared to the bandwidth currently available to temporally smoothed glass-based laser systems.

Korotkevich, Alexander O.↗
DOE OSTI2015

Laser parametric instability experiments of a 3ω, 15 kJ, 6-ns laser pulse in gas-filled hohlraums at the Ligne d'Intégration Laser facility

Experimental investigation of stimulated Raman (SRS) and Brillouin (SBS) scattering have been obtained at the Ligne-d'Intégration-Laser facility (LIL, CEA-Cesta, France). The parametric instabilities (LPI) are driven by firing four laser beamlets (one quad) into millimeter size, gas-filled hohlraum targets. A quad delivers energy on target of 15 kJ at 3ω in a 6-ns shaped laser pulse. The quad is focused by means of 3ω gratings and is optically smoothed with a kinoform phase plate and with smoothing by spectral dispersion-like 2 GHz and/or 14 GHz laser bandwidth. Open- and closed-geometry hohlraums have been used, all being filled with 1-atm, neo-pentane (C{sub 5}H{sub 12}) gas. For SRS and SBS studies, the light backscattered into the focusing optics is analyzed with spectral and time resolutions. Near-backscattered light at 3ω and transmitted light at 3ω are also monitored in the open geometry case. Depending on the target geometry (plasma length and hydrodynamic evolution of the plasma), it is shown that, at maximum laser intensity about 9 × 10{sup 14} W/cm{sup 2}, Raman reflectivity noticeably increases up to 30% in 4-mm long plasmas while SBS stays below 10%. Consequently, laser transmission through long plasmas drops to about 10% of incident energy. Adding 14 GHz bandwidth to the laser always reduces LPI reflectivities, although this reduction is not dramatic.

2-METHYLBUTANE↗
DOE OSTI2015

Numerical modeling of pulsed laser-material interaction and of laser plume dynamics

We have developed two-dimensional Arbitrary Lagrangian Eulerian (ALE) code which is used to study the physical processes, the plasma absorption, the crater profile, and the temperature distribution on metallic target and below the surface. The ALE method overcomes problems with Lagrangian moving mesh distortion by mesh smoothing and conservative quantities remapping from Lagrangian mesh to smoothed one. A new second order accurate diffusion solver has been implemented for the thermal conduction and radiation transport on distorted mesh. The results of numerical simulation of pulsed laser ablation are presented. The influences of different processes, such as time evolution of the surface temperature, interspecies interactions (elastic collisions, recombination-dissociation reaction), interaction with an ambient gas are examined. The study presents particular interest for the analysis of experimental results obtained during pulsed laser ablation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

KOH based selective wet chemical etching of AlN, Al{sub x}Ga{sub 1−x}N, and GaN crystals: A way towards substrate removal in deep ultraviolet-light emitting diode

A controllable and smooth potassium hydroxide-based wet etching technique was developed for the AlGaN system. High selectivity between AlN and Al{sub x}Ga{sub 1−x}N (up to 12×) was found to be critical in achieving effective substrate thinning or removal for AlGaN-based deep ultraviolet light emitting diodes, thus increasing light extraction efficiency. The mechanism of high selectivity of AlGaN as a function of Al composition can be explained as related to the formation and dissolution of oxide/hydroxide on top of N-polar surface. Cross-sectional transmission electron microscopic analysis served as ultimate proof that these hillocks were not related to underlying threading dislocations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Surface and subsurface cracks characteristics of single crystal SiC wafer in surface machining

Different machining processes were used in the single crystal SiC wafer machining. SEM was used to observe the surface morphology and a cross-sectional cleavages microscopy method was used for subsurface cracks detection. Surface and subsurface cracks characteristics of single crystal SiC wafer in abrasive machining were analysed. The results show that the surface and subsurface cracks system of single crystal SiC wafer in abrasive machining including radial crack, lateral crack and the median crack. In lapping process, material removal is dominated by brittle removal. Lots of chipping pits were found on the lapping surface. With the particle size becomes smaller, the surface roughness and subsurface crack depth decreases. When the particle size was changed to 1.5µm, the surface roughness Ra was reduced to 24.0nm and the maximum subsurface crack was 1.2µm. The efficiency of grinding is higher than lapping. Plastic removal can be achieved by changing the process parameters. Material removal was mostly in brittle fracture when grinding with 325# diamond wheel. Plow scratches and chipping pits were found on the ground surface. The surface roughness Ra was 17.7nm and maximum subsurface crack depth was 5.8 µm. When grinding with 8000# diamond wheel, the material removal was in plastic flow. Plastic scratches were found on the surface. A smooth surface of roughness Ra 2.5nm without any subsurface cracks was obtained. Atomic scale removal was possible in cluster magnetorheological finishing with diamond abrasive size of 0.5 µm. A super smooth surface eventually obtained with a roughness of Ra 0.4nm without any subsurface crack.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Electronic chemical response indexes at finite temperature in the canonical ensemble

Assuming that the electronic energy is given by a smooth function of the number of electrons and within the extension of density functional theory to finite temperature, the first and second order chemical reactivity response functions of the Helmholtz free energy with respect to the temperature, the number of electrons, and the external potential are derived. It is found that in all cases related to the first or second derivatives with respect to the number of electrons or the external potential, there is a term given by the average of the corresponding derivative of the electronic energy of each state (ground and excited). For the second derivatives, including those related with the temperature, there is a thermal fluctuation contribution that is zero at zero temperature. Thus, all expressions reduce correctly to their corresponding chemical reactivity expressions at zero temperature and show that, at room temperature, the corrections are very small. When the assumption that the electronic energy is given by a smooth function of the number of electrons is replaced by the straight lines behavior connecting integer values, as required by the ensemble theorem, one needs to introduce directional derivatives in most cases, so that the temperature dependent expressions reduce correctly to their zero temperature counterparts. However, the main result holds, namely, at finite temperature the thermal corrections to the chemical reactivity response functions are very small. Consequently, the present work validates the usage of reactivity indexes calculated at zero temperature to infer chemical behavior at room and even higher temperatures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Engineering the mechanical properties of ultrabarrier films grown by atomic layer deposition for the encapsulation of printed electronics

Direct deposition of barrier films by atomic layer deposition (ALD) onto printed electronics presents a promising method for packaging devices. Films made by ALD have been shown to possess desired ultrabarrier properties, but face challenges when directly grown onto surfaces with varying composition and topography. Challenges include differing nucleation and growth rates across the surface, stress concentrations from topography and coefficient of thermal expansion mismatch, elastic constant mismatch, and particle contamination that may impact the performance of the ALD barrier. In such cases, a polymer smoothing layer may be needed to coat the surface prior to ALD barrier film deposition. We present the impact of architecture on the performance of aluminum oxide (Al{sub 2}O{sub 3})/hafnium oxide (HfO{sub 2}) ALD nanolaminate barrier films deposited on fluorinated polymer layer using an optical calcium (Ca) test under damp heat. It is found that with increasing polymer thickness, the barrier films with residual tensile stress are prone to cracking resulting in rapid failure of the Ca sensor at 50 °C/85% relative humidity. Inserting a SiN{sub x} layer with residual compressive stress between the polymer and ALD layers is found to prevent cracking over a range of polymer thicknesses with more than 95% of the Ca sensor remaining after 500 h of testing. These results suggest that controlling mechanical properties and film architecture play an important role in the performance of direct deposited ALD barriers.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Accessing a growth window for SrVO{sub 3} thin films

Stoichiometric SrVO{sub 3} thin films were grown over a range of cation fluxes on (001) (La{sub 0.3}Sr{sub 0.7})(Al{sub 0.65}Ta{sub 0.35})O{sub 3} substrates using hybrid molecular beam epitaxy, where a thermal effusion cell was employed to generate a Sr flux and V was supplied using the metal-organic precursor vanadium oxytriisopropoxide (VTIP). By systematically varying the VTIP flux while keeping the Sr flux constant, a range of flux ratios were discovered in which the structural and electronic properties of the SrVO{sub 3} films remained unaltered. The intrinsic film lattice parameter and residual resistivity were found to be the smallest inside the growth window, indicating the lowest defect concentration of the films, and rapidly increased for cation flux ratios deviating from ideal growth condition. Reflection high-energy electron diffraction showed that films grown within this range had smooth surfaces and diffraction patterns were free of additional spots, while otherwise the growing surface was rough and contained additional crystalline phases. Results show the existence of a SrVO{sub 3} growth window at sufficiently high growth temperature, in which high-quality, stoichiometric films can be grown in a robust, highly reproducible manner that is invulnerable to unintentional flux variation.

36 MATERIALS SCIENCE↗