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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 1,495 records · Page 83
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Investigation of the Ti/MgCl{sub 2} interface on a Si(111) 7 x 7 substrate

Photoelectron spectroscopy with synchrotron radiation, low energy electron diffraction, and ion-scattering spectroscopy were used in order to study the Ti/MgCl{sub 2} interface grown on an atomically clean Si(111) 7 x 7 substrate. A series of high resolution spectra after deposition of a thick MgCl{sub 2} layer, step by step deposition of Ti and gradual annealing, indicated a very reactive interface even at room temperature. Strong interaction between the incoming Ti atoms and the MgCl{sub 2} layer, leads to the formation of Ti{sup 2+} and Ti{sup 4+} oxidation states. The interfacial interaction continues even at multilayer Ti coverage mainly by the partial disruption of Mg-Cl bonds and the formation of Ti-Cl sites, rendering this interface a very promising UHV-compatible model of a pre-catalyst for olefin polymerization. After the final annealing, the MgCl{sub 2} multilayers desorb while Ti remains on the surface forming a silicide layer on which Cl and Mg atoms are attached.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Intermetallic compound formation at Cu-Al wire bond interface

Intermetallic compound (IMC) formation and evolution at Cu-Al wire bond interface were studied using focused ion beam /scanning electron microscopy, transmission electron microscopy (TEM)/energy dispersive x-ray spectroscopy (EDS), nano beam electron diffraction (NBED) and structure factor (SF) calculation. It was found that discrete IMC patches were formed at the Cu/Al interface in as-packaged state and they grew toward Al pad after high temperature storage (HTS) environment at 150 Degree-Sign C. TEM/EDS and NBED results combined with SF calculation revealed the evidence of metastable {theta} Prime -CuAl{sub 2} IMC phase (tetragonal, space group: I4m2, a = 0.404 nm, c= 0.580 nm) formed at Cu/Al interfaces in both of the as-packaged and the post-HTS samples. Two feasible mechanisms for the formation of the metastable {theta} Prime -CuAl{sub 2} phase are discussed based on (1) non-equilibrium cooling of wire bond that is attributed to highly short bonding process time and (2) the epitaxial relationships between Cu and {theta} Prime -CuAl{sub 2}, which can minimize lattice mismatch for {theta} Prime -CuAl{sub 2} to grow on Cu.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Energy level alignment at the interfaces between typical electrodes and nucleobases: Al/adenine/indium-tin-oxide and Al/thymine/indium-tin-oxide

We investigated the interfacial electronic structures of Al/adenine/indium-tin-oxide (ITO) and Al/thymine/ITO using in situ ultraviolet and x-ray photoemission spectroscopy and density functional theory calculations. Adenine shows both an interface dipole and level bending, whereas thymine shows only an interface dipole in contact with ITO. In addition, thymine possesses a larger ionization energy than adenine. These are understood with delocalized {pi} states confirmed with theoretical calculations. For the interface between nucleobases and Al, both nucleobases show a prominent reduction of the electron injection barrier from Al to each base in accordance with a downward level shift.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

An XPS study on the chemical bond structure at the interface between SiO{sub x}N{sub y} and N doped polyethylene terephthalate

The super-thin silicon oxynitride (SiO{sub x}N{sub y}) films were deposited onto the N doped polyethylene terephthalate (PET) surface. Varying the N doping parameters, the different chemical bond structures were obtained at the interface between the SiO{sub x}N{sub y} film and the PET surface. X-ray photoelectron spectra results showed that at the initial stage of SiO{sub x}N{sub y} film growth, the C=N bonds could be broken and C-N-Si crosslink bonds could be formed at the interface of SiO{sub x}N{sub y}/PET, which C=N bonds could be formed onto the PET surface during the N doping process. At these positions, the SiO{sub x}N{sub y} film could be crosslinked well onto the PET surface. Meanwhile, the doped N could crosslink the [SiO{sub 4}] and [SiN{sub 4}] tetrahedrons, which could easily form the dense layer structure at the initial stage of SiO{sub x}N{sub y} film growth, instead of the ring and/or chain structures of [SiO{sub 4}] tetrahedrons crosslinked by O. Finally, from the point of applying SiO{sub x}N{sub y}/PET complex as the substrate, the present work reveals a simple way to crosslink them, as well as the crosslink model and physicochemical mechanism happened at the interface of complex.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Weakly nonlinear incompressible Rayleigh-Taylor instability growth at cylindrically convergent interfaces

A weakly nonlinear (WN) model has been developed for the incompressible Rayleigh-Taylor instability (RTI) in cylindrical geometry. The transition from linear to nonlinear growth is analytically investigated via a third-order solutions for the cylindrical RTI initiated by a single-mode velocity perturbation. The third-order solutions can depict the early stage of the interface asymmetry due to the bubble-spike formation, as well as the saturation of the linear (exponential) growth of the fundamental mode. The WN results in planar RTI [Wang et al., Phys. Plasmas 19, 112706 (2012)] are recovered in the limit of high-mode number perturbations. The difference between the WN growth of the RTI in cylindrical geometry and in planar geometry is discussed. It is found that the interface of the inward (outward) development spike/bubble is extruded (stretched) by the additional inertial force in cylindrical geometry compared with that in planar geometry. For interfaces with small density ratios, the inward growth bubble can grow fast than the outward growth spike in cylindrical RTI. Moreover, a reduced formula is proposed to describe the WN growth of the RTI in cylindrical geometry with an acceptable precision, especially for small-amplitude perturbations. Using the reduced formula, the nonlinear saturation amplitude of the fundamental mode and the phases of the Fourier harmonics are studied. Thus, it should be included in applications where converging geometry effects play an important role, such as the supernova explosions and inertial confinement fusion implosions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Wetting at the BaTiO{sub 3}/Pt interface

Using density functional theory, we analyze the wetting conditions for Pt on the (001) surface of ferroelectric BaTiO{sub 3} (BTO). We estimate the surface energy of (100), (110), and (111) Pt to be 2.42, 2.49, and 2.00 J/m{sup 2}, respectively. We find the BTO surface energy to vary between 0.26 and 2.28 J/m{sup 2} depending on termination, polarization, and chemical environment. The interface energy between TiO{sub 2}-terminated out-of-plane polarized BTO and (100) Pt is found to be between 1.64 and 2.62 J/m{sup 2}, indicating that (100) Pt cannot wet BTO for this interface. A similar result is found for an interface with (110) Pt. Cross-sectional transmission electron microscopy of Pt films grown on BTO by molecular beam epitaxy with a low flux at high deposition temperature shows Volmer-Weber islands, consistent with first principles calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Gold-titania interface toughening and thermal conductance enhancement using an organophosphonate nanolayer

We demonstrate that a mercaptan-terminated organophosphonate nanolayer at gold-titania interfaces can give rise to two- to three-fold enhancement in the interfacial fracture toughness and thermal conductance. Electron spectroscopy reveals that interfacial delamination occurs at the metal-molecule interface near the gold-sulfur bonds, consistent with density functional theory calculations of bond energies. Qualitative correlation between interfacial fracture toughness and bond energies suggest that organophosphonate nanolayers are resilient to humidity-induced degradation. These results, and the versatility of organophosphonates as surface functionalization agents for technologically relevant materials, unlock uncharted avenues for molecular engineering of interfaces in materials and devices for a variety of applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Direct observation of charge transfer region at interfaces in graphene devices

Nanoscale spectromicroscopic characterizing technique is indispensable for realization of future high-speed graphene transistors. Highly spatially resolved soft X-ray photoelectron microscopy measurements have been performed using our '3D nano-ESCA' (three-dimensional nanoscale electron spectroscopy for chemical analysis) equipment in order to investigate the local electronic states at interfaces in a graphene device structure. We have succeeded in detecting a charge transfer region at the graphene/metal-electrode interface, which extends over {approx}500 nm with the energy difference of 60 meV. Moreover, a nondestructive depth profiling reveals the chemical properties of the graphene/SiO{sub 2}-substrate interface.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Molecular dynamics simulations of n-hexane at 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide interface

Molecular dynamics simulations of n-hexane adsorbed onto the interface of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([bmim][Tf{sub 2}N]) are performed at three n-hexane surface densities, ranged from 0.7 to 2.3 {mu}mol/m{sup 2} at 300 K. For [bmim][Tf{sub 2}N] room-temperature ionic liquid, we use a non-polarizable all-atom force field with the partial atomic charges based on ab initio calculations for the isolated ion pair. The net charges of the ions are {+-}0.89e, which mimics the anion to cation charge transfer and polarization effects. The OPLS-AA force field is employed for modeling of n-hexane. The surface tension is computed using the mechanical route and its value decreases with increase of the n-hexane surface density. The [bmim][Tf{sub 2}N]/n-hexane interface is analyzed using the intrinsic method, and the structural and dynamic properties of the interfacial, sub-interfacial, and central layers are computed. We determine the surface roughness, global and intrinsic density profiles, and orientation ordering of the molecules to describe the structure of the interface. We further compute the survival probability, normal and lateral self-diffusion coefficients, and re-orientation correlation functions to elucidate the effects of n-hexane on dynamics of the cations and anions in the layers.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Nanoparticles at liquid interfaces: Rotational dynamics and angular locking

Nanoparticles with different surface morphologies that straddle the interface between two immiscible liquids are studied via molecular dynamics simulations. The methodology employed allows us to compute the interfacial free energy at different angular orientations of the nanoparticle. Due to their atomistic nature, the studied nanoparticles present both microscale and macroscale geometrical features and cannot be accurately modeled as a perfectly smooth body (e.g., spheres and cylinders). Under certain physical conditions, microscale features can produce free energy barriers that are much larger than the thermal energy of the surrounding media. The presence of these energy barriers can effectively “lock” the particle at specific angular orientations with respect to the liquid-liquid interface. This work provides new insights on the rotational dynamics of Brownian particles at liquid interfaces and suggests possible strategies to exploit the effects of microscale features with given geometric characteristics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Direct measurement of band offset at the interface between CdS and Cu{sub 2}ZnSnS{sub 4} using hard X-ray photoelectron spectroscopy

We directly and non-destructively measured the valence band offset at the interface between CdS and Cu{sub 2}ZnSnS{sub 4} (CZTS) using hard X-ray photoelectron spectroscopy (HAXPES), which can measure the electron state of the buried interface because of its large analysis depth. These measurements were made using the following real devices; CZTS(t = 700 nm), CdS(t = 100 nm)/CZTS(t = 700 nm), and CdS(t = 5 nm)/CZTS(t = 700 nm) films formed on Mo coated glass. The valence band spectra were measured by HAXPES using an X-ray photon energy of 8 keV. The value of the valence band offset at the interface between CdS and CZTS was estimated as 1.0 eV by fitting the spectra. The conduction band offset could be deduced as 0.0 eV from the obtained valence band offset and the band gap energies of CdS and CZTS.

36 MATERIALS SCIENCE↗
DOE OSTI2013

A transition in mechanisms of size dependent electrical transport at nanoscale metal-oxide interfaces

As device miniaturization approaches nanoscale dimensions, interfaces begin to dominate electrical properties. Here the system archetype Au/SrTiO{sub 3} is used to examine the origin of size dependent transport properties along metal-oxide interfaces. We demonstrate that a transition between two classes of size dependent electronic transport mechanisms exists, defined by a critical size ε. At sizes larger than ε an edge-related tunneling effect proportional to 1/D (the height of the supported Au nanoparticle) is observed; interfaces with sizes smaller than ε exhibit random fluctuations in current. The ability to distinguish between these mechanisms is important to future developments in nanoscale device design.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Improved lateral photovoltaic effect of Ti and carbon films by interface modification with single-walled carbon nanotubes

An efficiently improved lateral photovoltaic effect (LPE) has been successfully observed in Ti/Si and amorphous carbon (a-C) film/Si structures by introducing single-walled carbon nanotubes (SWNTs) as modifying interface instead of native SiO{sub 2} layer grown on Si substrate. The largest lateral photovoltage (LPV) position sensitivity achieved is 67.02 mV/mm for the Ti/Si system and 2.23 mV/mm for the a-C/Si system. This corresponds to an improvement of 40% for the Ti/Si system and 2600% for the a-C/Si system. Besides, the SWNTs modified interface also induced a well-marked shift of optimal film thickness in both materials. An additional novel phenomenon is that the directly observed LPV is much larger in SWNTs/Si system compared to the improved a-C/SWNTs/Si structure. A mechanism based on the change of interface states is given to interpret these results, which not only suggests a new common modulation method for LPE, but also a new potential application of SWNTs for photo-electronic device.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Elastic strains at interfaces in InAs/AlSb multilayer structures for quantum cascade lasers

InAs/AlSb multilayers similar to those used in quantum cascade lasers have been grown by molecular beam epitaxy on (001) InAs substrates. Elastic strain is investigated by high resolution transmission electron microscopy. Thin interfacial regions with lattice distortions significantly different from the strain of the AlSb layers themselves are revealed from the geometrical phase analysis. Strain profiles are qualitatively compared to the chemical contrast of high angle annular dark field images obtained by scanning transmission electron microscopy. The strain and chemical profiles are correlated with the growth sequences used to form the interfaces. Tensile strained AlAs-like interfaces tend to form predominantly due to the high thermal stability of AlAs. Strongly asymmetric interfaces, AlAs-rich and (Al, In)Sb, respectively, can also be achieved by using appropriate growth sequences.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

A futuristic approach towards interface layer modifications for improved efficiency in inverted organic solar cells

Inverted polymer Solar Cells of the classical poly (3-hexylthiophene) (P3HT):(6,6)-phenyl-C{sub 61}butyric acid methyl ester (PC{sub 61}BM) blend on indium tin oxide substrates were fabricated, which shows improved device performance, by using a facile solution–processed ZnO-polyelectrolytes [poly (diallyldimethylammonium chloride) (PDADMAC), Poly (acrylic acid sodium salt) (PAS), poly (4-styrenesulfonic acid) (PSS), and Polyvinylpyrrolidone (PVP)] nanocomposite as a cathode interface layer compared to devices using pristine ZnO as cathode buffer layer in ambient conditions. The devices with different combinations of polyelectrolyte with ZnO show different improvements in the device efficiency. The combinations of ZnO with PVP and PDADMAC show highest amount of improvements in the efficiency by a factor of ∼17–19. The improvement of the efficiency may be due to various phenomena, such as the passivation of ZnO surface as well as bulk traps, work function modification, improved energy level alignment, improved electronic coupling of the inorganic/organic interface, improved light harvesting, and decrease of surface as well as bulk charge recombination in the device. The introduction of polyelectrolyte into ZnO inhibits the aggregation of ZnO nanoparticles yielding the large area ZnO nanoclusters; and hence, forming the uniform film of ZnO resulting in the modifications of morphology as well as electronic structure of ZnO-polyelectrolyte nano-composite favouring better electronic coupling between cathode and active layer and hence enhancing the current and, consequently, the efficiency. This simple low temperature ZnO-polyelectrolyte nanocomposite based protocol proposed for cathode interface layer modification may be very much useful for roll to roll industrial manufacturing of organic solar cells.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Enhancement of the magnetic interfacial exchange energy at a specific interface in NiFe/CoO/Co trilayer thin films via ion-beam modification

A series of ferromagnetic Ni{sub 80}Fe{sub 20}(55 nm)/antiferromagnetic CoO (25 to 200 nm)/ferromagnetic Co (55 nm)/SiO{sub 2}(substrate) trilayer thin films were fabricated by ion-beam assisted deposition in order to understand the role of ion beam modification on the interfacial and interlayer coupling. The microstructural study using transmission electron microscopy, X-ray reflectometry, and polarised neutron reflectometry showed that ion-beam modification during the deposition process led to an oxygen-rich Co/CoO nanocomposite interface region at the bottom layer. This interface caused a high exchange bias field for the ferromagnetic cobalt. However, the exchange bias for top permalloy ferromagnet remained low, in line with expectations from the literature for the typical interfacial energy. This suggest that the ion-beam enhancement of the magnetic exchange bias is localized to the Co/CoO interface where local microstructural effects provide the dominant mechanism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Thermal interface conductance across a graphene/hexagonal boron nitride heterojunction

We measure thermal transport across a graphene/hexagonal boron nitride (h-BN) interface by electrically heating the graphene and measuring the temperature difference between the graphene and BN using Raman spectroscopy. Because the temperature of the graphene and BN are measured optically, this approach enables nanometer resolution in the cross-plane direction. A temperature drop of 60 K can be achieved across this junction at high electrical powers (14 mW). Based on the temperature difference and the applied power data, we determine the thermal interface conductance of this junction to be 7.4 × 10{sup 6} Wm{sup −2}K{sup −1}, which is below the 10{sup 7}–10{sup 8} Wm{sup −2}K{sup −1} values previously reported for graphene/SiO{sub 2} interface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Hetero-epitaxial EuO interfaces studied by analytic electron microscopy

With nearly complete spin polarization, the ferromagnetic semiconductor europium monoxide could enable next-generation spintronic devices by providing efficient ohmic spin injection into silicon. Spin injection is greatly affected by the quality of the interface between the injector and silicon. Here, we use atomic-resolution scanning transmission electron microscopy in conjunction with electron energy loss spectroscopy to directly image and chemically characterize a series of EuO|Si and EuO|YAlO{sub 3} interfaces fabricated using different growth conditions. We identify the presence of europium silicides and regions of disorder at the EuO|Si interfaces, imperfections that could significantly reduce spin injection efficiencies via spin-flip scattering.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗