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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Surface-plasmon enhanced photoemission of a silver nano-patterned photocathode

Here, nano-patterned photocathodes (NPC) take advantage of plasmonic effects to resonantly increase absorption of light and localize electromagnetic field intensity on metal surfaces leading to surface-plasmon enhanced photoemission. In this paper, we report the status of NPC research at UCLA including in particular the optimization of the dimensions of a nanohole array on a silver wafer to enhance plasmonic response at 800 nm light, the development of a spectrally-resolved reflectivity measurement setup for quick nanopattern validation, and of a novel cathode plug to enable high power tests of NPCs on single crystal substrates in a high gradient radiofrequency gun.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2025

Sub-tesla On-Chip Nanomagnetic Metamaterial Platform for Angle-Resolved Photoemission Spectroscopy

Magnetically controlled states in quantum materials are central to their unique electronic and magnetic properties. However, direct momentum-resolved visualization of these states via angle-resolved photoemission spectroscopy (ARPES) has been hindered by the disruptive effect of magnetic fields on photoelectron trajectories. Here, we introduce an in situ method that is, in principle, capable of applying magnetic fields up to 1 T. This method uses substrates composed of nanomagnetic metamaterial arrays with alternating polarity. Such substrates can generate strong, homogeneous, and spatially confined fields applicable to samples with thicknesses up to the micron scale, enabling ARPES measurements under magnetic fields with minimal photoelectron trajectory distortion. Here, we demonstrate this minimal distortion with ARPES data taken on monolayer graphene. Our method paves the way for probing magnetic field-dependent electronic structures and studying field-tunable quantum phases with state-of-the-art energy-momentum resolutions.

Magnetic properties↗
DOE OSTI2015

Band Edge Energetics of Heterostructured Nanorods: Photoemission Spectroscopy and Waveguide Spectroelectrochemistry of Au-Tipped CdSe Nanorod Monolayers

Conduction and valence band energies ( E CB , E VB ) for CdSe nanorods (NRs) functionalized with Au nanoparticle (NP) tips are reported here, referenced to the vacuum scale. We use (a) UV photoemission spectroscopy (UPS) to measure E VB for NR films, utilizing advanced approaches to secondary electron background correction, satellite removal to enhance spectral contrast, and correction for shifts in local vacuum levels; and (b) waveguide-based spectroelectrochemistry to measure E CB from onset potentials for electron injection into NR films tethered to ITO. For untipped CdSe NRs, both approaches show E VB = 5.9–6.1 eV and E CB = 4.1–4.3 eV. Addition of Au tips alters the NR band edge energies and introduces midgap states, in ways that are predicted to influence the efficiency of these nanomaterials as photoelectrocatalysts. UPS results show that Au tipping shifts E VB closer to vacuum by up to 0.4 eV, shifts the apparent Fermi energy toward the middle of the band gap, and introduces additional states above E VB . Spectroelectrochemical results determine these trends: Au tipping shifts E CB closer to vacuum, by 0.4–0.6 eV, and introduces midgap states below E CB , which are assigned as metal–semiconductor interface (MSI) states. Characterization of these band edge energies and understanding the origins of MSI states is needed to design energy conversion systems with proper band alignment between the light absorbing NR, the NP catalyst, and solution electron donors and acceptors. Furthermore, the complementary characterization protocols presented here should be applicable to a wide variety of thin films of heterogeneous photoactive nanomaterials, aiding in the identification of the most promising material combinations for photoelectrochemical energy conversion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Experimental Determination of the Ionization Energies of MoSe 2 , WS 2 , and MoS 2 on SiO 2 Using Photoemission Electron Microscopy

Here, the values of the ionization energies of transition metal dichalcogenides (TMDs) are needed to assess their potential usefulness in semiconductor heterojunctions for high-performance optoelectronics. Here, we report on the systematic determination of ionization energies for three prototypical TMD monolayers (MoSe 2 , WS 2 , and MoS 2 ) on SiO 2 using photoemission electron microscopy with deep ultraviolet illumination. The ionization energy displays a progressive decrease from MoS 2 , to WS 2 , to MoSe 2 , in agreement with predictions of density functional theory calculations. Combined with the measured energy positions of the valence band edge at the Brillouin zone center, we deduce that, in the absence of interlayer coupling, a vertical heterojunction comprising any of the three TMD monolayers would form a staggered (type-II) band alignment. This band alignment could give rise to long-lived interlayer excitons that are potentially useful for valleytronics or efficient electron–hole separation in photovoltaics.

36 MATERIALS SCIENCE↗
DOE OSTI2010

New ambient pressure photoemission endstation at Advanced Light Source beamline 9.3.2

During the past decade, the application of ambient pressure photoemission spectroscopy (APPES) has been recognized as an important in situ tool to study environmental and materials science, energy related science, and many other fields. Several APPES endstations are currently under planning or development at the USA and international light sources, which will lead to a rapid expansion of this technique. The present work describes the design and performance of a new APPES instrument at the Advanced Light Source beamline 9.3.2 at Lawrence Berkeley National Laboratory. This new instrument, Scienta R4000 HiPP, is a result of collaboration between Advanced Light Source and its industrial partner VG-Scienta. The R4000 HiPP provides superior electron transmission as well as spectromicroscopy modes with 16 {mu}m spatial resolution in one dimension and angle-resolved modes with simulated 0.5 deg. angular resolution at 24 deg. acceptance. Under maximum transmission mode, the electron detection efficiency is more than an order of magnitude better than the previous endstation at beamline 9.3.2. Herein we describe the design and performance of the system, which has been utilized to record spectra above 2 mbar.

43 PARTICLE ACCELERATORS↗
DOE OSTI2010

Standing-wave excited soft x-ray photoemission microscopy: application to Co microdot magnetic arrays

We demonstrate the addition of depth resolution to the usual two-dimensional images in photoelectron emission microscopy (PEEM), with application to a square array of circular magnetic Co microdots. The method is based on excitation with soft x-ray standing-waves generated by Bragg reflection from a multilayer mirror substrate. Standing wave is moved vertically through sample simply by varying the photon energy around the Bragg condition. Depth-resolved PEEM images were obtained for all of the observed elements. Photoemission intensities as functions of photon energy were compared to x-ray optical calculations in order to quantitatively derive the depth-resolved film structure of the sample.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Ultrahigh-resolution spin-resolved photoemission spectrometer with a mini Mott detector

We have developed an ultrahigh-resolution spin-resolved photoemission spectrometer with a highly efficient mini Mott detector and an intense xenon plasma discharge lamp. The spectrometer achieves the energy resolutions of 0.9 and 8 meV for non-spin-resolved and spin-resolved modes, respectively. Three-dimensional spin-polarization is determined by using a 90 deg. electron deflector situated before the Mott detector. The performance of spectrometer is demonstrated by observation of a clear Rashba splitting of the Bi(111) surface states.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

High-resolution core-level photoemission study of CF{sub 4}-treated Gd{sub 2}O{sub 3}(Ga{sub 2}O{sub 3}) gate dielectric on Ge probed by synchrotron radiation

High-resolution core-level photoemission analysis using synchrotron radiation was used to investigate the superior electrical performance of aGa{sub 2}O{sub 3}(Gd{sub 2}O{sub 3}) gate dielectric on Ge(001) after CF{sub 4} treatment. Prior to the treatment, a thin germanate-like oxide layer that formed at the interface prevented Ge from diffusing to the surface. The Ge surface retained a small amount of buckled dimers from the as-grown sample. The buckled dimers were quickly removed by CF{sub 4} plasma treatment followed by an annealing process, resulting in a more uniform interface than that of the as-grown sample. The detailed interfacial electronic structure for the untreated and treated samples are presented.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Fingerprints of the hydrogen bond in the photoemission spectra of croconic acid condensed phase: An x-ray photoelectron spectroscopy and ab-initio study

The electronic structure of Croconic Acid in the condensed phase has been studied by comparing core level and valence band x-ray photoelectron spectroscopy experiments and first principles density functional theory calculations using the Heyd-Scuseria-Ernzerhof screened hybrid functional and the GW approximation. By exploring the photoemission spectra for different deposition thicknesses, we show how the formation of the hydrogen bond network modifies the O 1s core level lineshape. Moreover, the valence band can be explained only if the intermolecular interactions are taken into account in the theoretical approach.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Electric Field Penetration in Au/Nb:SrTiO3 Schottky Junctions Probed by Bias-Dependent Internal Photoemission

Electric field penetration into the metallic side of a Schottky junction is in principle a universal phenomenon, the magnitude of which increases with the semiconductor permittivity. Here, we quantitatively probe this effect using bias-dependent internal photoemission spectroscopy at the Schottky junction between a large dielectric permittivity semiconductor SrTiO{sub 3} and gold. A clear linear reduction of the barrier height with increasing interface electric field was observed, highlighting the importance of field penetration into the gold. The interfacial permittivity of SrTiO{sub 3} at the interface is reduced from the bulk value, reflecting intrinsic suppression at the interface.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Element-specific study of epitaxial NiO/Ag/CoO/Fe films grown on vicinal Ag(001) using photoemission electron microscopy

NiO/Ag/CoO/Fe single crystalline films are grown epitaxially on a vicinal Ag(001) substrate using molecular beam epitaxy and investigated by photoemission electron microscopy. We find that after zero-field cooling, the in-plane Fe magnetization switches from parallel to perpendicular direction of the atomic steps of the vicinal surface at thinner CoO thickness but remains in its original direction parallel to the steps at thicker CoO thickness. CoO and NiO domain imaging result shows that both CoO/Fe and NiO/CoO spins are perpendicularly coupled, suggesting that the Fe magnetization switching may be associated with the rotatable-frozen spin transition of the CoO film.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Photon Detector For Inverse Photoemission Spectroscopy With Improved Energy Resolution

We present the results from newly designed and fabricated double window photon detector to improve the overall energy resolution for inverse photoemission spectroscopy (IPES). This simple design allows us to introduce an absorption gas (Krypton) to decrease the band-width of the energy selective photon detector and thus improve the resolution. Resonance absorption line of Kr of wavelength of 123.6 nm was used. By fitting the Fermi edge of the IPES spectrum of silver, we find an overall energy resolution improved by 73 meV. The design is modular and ensures ease and safety of handling.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

Electronic Band Dispersion Of CeAg{sub 2}Ge{sub 2} Studied Using Angle Resolved Photoemission Spectroscopy

Angle resolved photoelectron spectroscopy has been used to determine the electronic band dispersion of CeAg{sub 2}Ge{sub 2} single crystal along the {Gamma}-Z direction. Normal emission photoelectron spectra for photon energy from 21 eV to 35 eV were used to obtain the band mapping. Four photoemission features are observed to show the dispersion. The experimental spectra have been interpreted with the help of calculations based on full potential linear augmented plane wave method using density functional theory. The bands that show dispersion along the {Gamma}-Z direction are mainly related to the Ce 4f states and the hybridized Ag 4d with Ge 4p states.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Photoemission induced bias in two-dimensional silicon pn junctions

Spectroscopic x-ray photoelectron emission microscopy was used to study the role of the pn junction on imaging of micron scale n- and p-doped silicon patterns epitaxially grown on p- and n-type substrates, respectively. In the closed n-doped patterns, contrast with respect to open patterns is observed in both work function and Si 2p binding energy. Reverse bias at the junction creates a shift of electrical potential induced by photoemission in the closed patterns. No shift is observed for p-doped patterns on n substrate, pointing to the importance of doping combination and pattern geometry in the contrast observed in electron microscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Polarity-dependent photoemission spectra of wurtzite-type zinc oxide

The polar surfaces of wurtzite-type zinc oxide (ZnO) were characterized by x-ray photoemission spectroscopy to identify the origin of the polarity dependence of the valence band spectra. A characteristic sub-peak always appeared in the valence band spectra of the (0001) face regardless of the surface preparation conditions. It also appeared in the valence band spectra of the (1012) face, but only when the photoelectron take-off angle was parallel to the c-axis of ZnO. Our analysis demonstrates that this take-off angle dependency originates not from the surface state, photoelectron diffraction, or the presence of surfactants but from the crystal polarity.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Direct imaging of both ferroelectric and antiferromagnetic domains in multiferroic BiFeO{sub 3} single crystal using x-ray photoemission electron microscopy

In this work, we propose to study the magnetic and ferroelectric configurations in ferroelectric multidomain BiFeO{sub 3} single crystals. Using x-ray (magnetic) linear dichroism in a photoemission electron microscope (X-PEEM), we are able to directly image both the antiferromagnetic and ferroelectric domains. We find that inside one single ferroelectric domain several antiferromagnetic domains coexist. This is different from what was observed on epitaxial thin films, where the ferroelectric domains perfectly match the antiferromagnetic ones, but also from previous neutron measurements on ferroelectric monodomain single-crystals for which one single antiferromagnetic domain was identified. This underlines the fundamental differences between thin films, bulk samples, and single versus ferroelectric multidomain samples.

36 MATERIALS SCIENCE↗
DOE OSTI2012

C-C bond unsaturation degree in monosubstituted ferrocenes for molecular electronics investigated by a combined near-edge x-ray absorption fine structure, x-ray photoemission spectroscopy, and density functional theory approach

We present the results of an experimental and theoretical investigation of monosubstituted ethyl-, vinyl-, and ethynyl-ferrocene (EtFC, VFC, and EFC) free molecules, obtained by means of synchrotron-radiation based C 1s photoabsorption (NEXAFS) and photoemission (C 1s XPS) spectroscopies, and density functional theory (DFT) calculations. Such a combined study is aimed at elucidating the role played by the C-C bond unsaturation degree of the substituent on the electronic structure of the ferrocene derivatives. Such substituents are required for molecular chemical anchoring onto relevant surfaces when ferrocenes are used for molecular electronics hybrid devices. The high resolution C 1s NEXAFS spectra exhibit distinctive features that depend on the degree of unsaturation of the hydrocarbon substituent. The theoretical approach to consider the NEXAFS spectrum made of three parts allowed to disentangle the specific contribution of the substituent group to the experimental spectrum as a function of its unsaturation degree. C 1s IEs were derived from the experimental data analysis based on the DFT calculated IE values for the different carbon atoms of the substituent and cyclopentadienyl (Cp) rings. Distinctive trends of chemical shifts were observed for the substituent carbon atoms and the substituted atom of the Cp ring along the series of ferrocenes. The calculated IE pattern was rationalized in terms of initial and final state effects influencing the IE value, with special regard to the different mechanism of electron conjugation between the Cp ring and the substituent, namely the {sigma}/{pi} hyperconjugation in EtFC and the {pi}-conjugation in VFC and EFC.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Deflection gating for time-resolved x-ray magnetic circular dichroism-photoemission electron microscopy using synchrotron radiation

In this paper, we present a newly developed gating technique for a time-resolving photoemission microscope. The technique makes use of an electrostatic deflector within the microscope's electron optical system for fast switching between two electron-optical paths, one of which is used for imaging, while the other is blocked by an aperture stop. The system can be operated with a switching time of 20 ns and shows superior dark current rejection. We report on the application of this new gating technique to exploit the time structure in the injection bunch pattern of the synchrotron radiation source BESSY II at Helmholtz-Zentrum Berlin for time-resolved measurements in the picosecond regime.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗