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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 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Ternary behavior and systematic nanoscale manipulation of domain structures in P3HT/PCBM/P3HT-b-PEO films

Nanophase separation plays a critical role in the performance of donor acceptor based organic photovoltaic (OPV) devices. Although post-fabrication annealing is often used to enhance OPV efficiency, the ability to exert precise control over phase separated domains and connectivity remains elusive. In this work, we use a diblock copolymer to systematically manipulate the domain sizes of an organic solar cell active layer at the nanoscale. More specifically, a poly(3-hexylthiophene)-bpoly( ethylene oxide) (P3HT-b-PEO) diblock copolymer with a low polydispersity index (PDI 1.3) is added to a binary blend of P3HT and 6,6-phenyl C61-butyric acid methyl ester (PCBM) at different concentrations (0 20 wt%). Energy-filtered TEM (EFTEM) results suggest systematic changes of P3HT distribution as a function of block copolymer compatibilizer concentration and thermal annealing. X-ray scattering and microscopy techniques are used to show that prior to annealing, active layer domain sizes do not change substantially as compatibilizer is added; however after thermal annealing, the domain sizes are significantly reduced as the amount of P3HT-b-PEO compatibilizer increases. The impact of compatibilizer is further rationalized through quantum density functional theory calculations. Overall, this work demonstrates the possibility of block copolymers to systematically manipulate the nanoscale domain-structure of blends used for organic photovoltaic devices. Furthermore, if coupled with efficient charge transport and collection (through judicious choice of block copolymer type and composition), this approach may contribute to further optimization of OPV devices.

14 SOLAR ENERGY↗
DOE OSTI2011

Spin injection from epitaxial Fe{sub 3}O{sub 4} films to ZnO films

The fully epitaxial Fe{sub 3}O{sub 4}/ZnO heterostructures were fabricated by reactive dc sputtering (Fe{sub 3}O{sub 4}) and rf sputtering (ZnO) on c-Al{sub 2}O{sub 3} substrates. The epitaxial relationship is verified to be Fe{sub 3}O{sub 4}(111)<110>(parallel sign)ZnO(0002)<1120> by {phi} scans. The nonlinear and rectifying properties of I-V curves were observed in the heterostructures. The transport mechanism across the interface between Fe{sub 3}O{sub 4} and ZnO is thermal emission and the Schottky barrier is calculated to be 0.51 eV. The magnetoresistance of the heterostructures is symmetric and depends on current. The spin polarization of the transport electrons from Fe{sub 3}O{sub 4} into ZnO is determined to be 28.5% at 30 K.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Improved photoresponsivity of semiconducting BaSi{sub 2} epitaxial films grown on a tunnel junction for thin-film solar cells

The highest photoresponsivity and an internal quantum efficiency exceeding 70% at 1.55 eV were achieved for 400 nm thick undoped n-type BaSi{sub 2} epitaxial layers formed on a n{sup +}-BaSi{sub 2}/p{sup +}-Si tunnel junction (TJ) on Si(111). The diffusion of Sb atoms was effectively suppressed by an intermediate polycrystalline Si layer grown by solid phase epitaxy, located between the TJ and undoped BaSi{sub 2} layers.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Surfaces of colloidal PbSe nanocrystals probed by thin-film positron annihilation spectroscopy

Positron annihilation lifetime spectroscopy and positron-electron momentum density (PEMD) studies on multilayers of PbSe nanocrystals (NCs), supported by transmission electron microscopy, show that positrons are strongly trapped at NC surfaces, where they provide insight into the surface composition and electronic structure of PbSe NCs. Our analysis indicates abundant annihilation of positrons with Se electrons at the NC surfaces and with O electrons of the oleic ligands bound to Pb ad-atoms at the NC surfaces, which demonstrates that positrons can be used as a sensitive probe to investigate the surface physics and chemistry of nanocrystals inside multilayers. Ab initio electronic structure calculations provide detailed insight in the valence and semi-core electron contributions to the positron-electron momentum density of PbSe. Both lifetime and PEMD are found to correlate with changes in the particle morphology characteristic of partial ligand removal.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Characterization of the temperature dependence of the thermoreflectance coefficient for conductive thin films

We describe a novel approach for calibration of the thermoreflectance coefficient, ideally suited for measurements in a vacuum thermostat, and present the high temperature thermoreflectance coefficients for several metals commonly encountered in electronic devices: gold, platinum, and aluminum. The effect of passivation on these metals is also examined, and we demonstrate the signal to noise ratio of a thermoreflectance measurement can be improved with informed selection of the dielectric layer thickness. Furthermore, the thermo-optic coefficients of the metals are extracted over a wide temperature range. The results presented here can be utilized in the optimization of experimental configurations for high temperature thermoreflectance imaging.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Enhanced third harmonic generation from the epsilon-near-zero modes of ultrathin films

We experimentally demonstrate efficient third harmonic generation from an indium tin oxide nanofilm (λ/42 thick) on a glass substrate for a pump wavelength of 1.4 μm. A conversion efficiency of 3.3 × 10{sup −6} is achieved by exploiting the field enhancement properties of the epsilon-near-zero mode with an enhancement factor of 200. This nanoscale frequency conversion method is applicable to other plasmonic materials and reststrahlen materials in proximity of the longitudinal optical phonon frequencies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Photo-induced refractive index and topographical surface gratings in functionalized nanocarbon solid film

We demonstrate that a single-walled carbon nanotube network noncovalently coupled with a pyrene-modified azo-benzene chromophore functions as a host matrix for a broad range of photo-orientation and photomechanical effects. The chromophore could be efficiently reoriented through repeated trans-cis-trans isomerization under linearly polarized 480 nm light, with Δn of 0.012 at 650 nm and fast characteristic rise-times of 0.12 s. Erasable phase diffraction gratings could also be written, with permanent surface relief gratings forming at sufficiently long irradiation times. In addition to demonstrating a mechanism for photo-manipulation of single-walled carbon nanotubes, these results show photo-orientation of chromophores in azo-functionalized single-walled carbon nanotube networks as a path towards the photosensitive tuning of the electrostatic environment of the nanotube.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Dynamical freezing, magnetic ordering, and the magnetocaloric effect in nanostructured Fe/Cu thin films

We report dynamical freezing of Fe nanocrystallites in a Cu matrix with magnetic ordering of an FeCu interfacial phase provides a unique window into the magnetocaloric effect. The FeCu alloy altered the Fe nanocrystallite surface atoms, and with a magnetic ordering temperature comparable to the dynamical freezing temperature of the nanocrystallites enabled Fe surface atoms to contribute to the overall magnetization. Tuning the amount of interfacial alloy resulted in the control of the magnetic ordering temperature and the magnetocaloric properties.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Integrating superconducting phase and topological crystalline quantum spin Hall effect in hafnium intercalated gallium film

Motivated by the growth of superconducting atomic hexagonal Ga layers on GaN surface we have calculated the electronic properties of Hf intercalated honeycomb Ga layers using first-principles theory. In contrast to the hexagonal Ga layers where substrate is necessary for their stability, we find the above structure to be dynamically stable in its freestanding form with small formation energy. In particular, six Dirac cones composed of Hf-d{sub xy}/d{sub x2-y2} orbitals are observed in the first Brillouin zone, slightly below the Fermi energy. Spin-orbit coupling opens a large band gap of 177 meV on these Dirac cones. By calculating its mirror Chern number, we demonstrate that this band gap is topologically nontrivial and protected by mirror symmetry. Such mirror symmetry protected band gaps are rare in hexagonal lattice. A large topological crystalline quantum spin Hall conductance σ{sub SH} ∼ −4 e{sup 2}/h is also revealed. Moreover, electron-phonon coupling calculations reveal that this material is superconducting with a transition temperature T{sub c} = 2.4 K, mainly contributed by Ga out-of-plane vibrations. Our results provide a route toward manipulating quantum spin Hall and superconducting behaviors in a single material which helps to realize Majorana fermions and topological superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Local imaging of diamagnetism in proximity-coupled niobium nanoisland arrays on gold thin films.

In this work, we study the effect of engineered disorder on the local magnetic response of proximity-coupled superconducting island arrays by comparing scanning superconducting quantum interference device (SQUID) susceptibility measurements to a model in which we treat the system as a network of one-dimensional (1D) superconductor-normal-metal-superconductor Josephson junctions, each with a Josephson coupling energy E-J determined by the junction length or distance between islands. We find that the disordered arrays exhibit a spatially inhomogeneous diamagnetic response which, for low local applied magnetic fields, is well described by this junction network model, and we discuss these results as they relate to inhomogeneous 2D superconductors. Our model of the static magnetic response of the arrays does not fully capture the onset of nonlinearity and dissipation with increasing applied field, as these effects are associated with vortex motion due to the dynamic nature of the scanning SQUID susceptometry measurement. This work demonstrates a model 2D superconducting system with engineered disorder, and it highlights the impact of dissipation on the local magnetic properties of 2D superconductors and Josephson junction arrays.

Bishop-Van Horn, Logan↗
DOE OSTI2012

Identification and Analysis of Distinct Features in Imaging Thin-Film Solar Cells: Preprint

Electroluminescence and photoluminescence (EL and PL) are two imaging techniques employed at NREL that are used to qualitatively evaluate solar cells. In this work, imaging lab-scale CdTe and CIGS devices provides information about small-area PV response, which will aid in determining the effects of non-uniformities on cell performance. EL, PL, and dark lock-in thermography signatures are firstcatalogued. Their responses to varying conditions are then studied. Further analysis includes acquiring spectral data, making microscopy measurements, and correlating luminescence to device performance. The goal of this work is to quantitatively determine non-uniformity effects on cell performance using rapid imaging techniques.

14 SOLAR ENERGY↗