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At least 1,765 records · Page 98
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Luminescence of phosphorus containing oxide materials: Crystalline SiO{sub 2}‐P and 3P{sub 2}O{sub 5}⋅7SiO{sub 2}; CaO⋅P{sub 2}O{sub 5}; SrO⋅P{sub 2}O{sub 5} glasses

Luminescence of phosphate glasses such as CaO⋅P{sub 2}O{sub 5} and SrO⋅P{sub 2}O{sub 5} is compared with that of phosphorus doped crystalline α-quartz and phosphosilicate glass with content 3P{sub 2}O{sub 5}⋅7SiO{sub 2}. Water and OH groups are found by IR spectra in these materials. The spectrum of luminescence contains many bands in the range 1.5 - 5.5 eV. The luminescence bands in UV range at 4.5-5 eV are similar in those materials. Decay duration in exponential approximation manifests a time constant about 37 ns. Also a component in μs range was detected. PL band of μs component is shifted to low energy with respect to that of ∼37 ns component. This shift is about 0.6 eV. It is explained as singlet-triplet splitting of excited state. Below 14 K increase of luminescence kinetics duration in μs range was observed and it was ascribed to zero magnetic field splitting of triplet excited state of the center. Yellow-red luminescence was induced by irradiation in phosphorus doped crystalline α-quartz, phosphosilicate glasses. The yellowl uminescence contains two bands at 600 and 740 nm. Their decay is similar under 193 nm laser and may be fitted with the first order fractal kinetics or stretched exponent. Thermally stimulated luminescence contains only band at 600 nm. The 248 nm laser excites luminescence at 740 nm according to intra center process with decay time constant about 4 ms at 9 K. Both type of luminescence UV and yellow were ascribed to different defects containing phosphorus. P-doped α-quartz sample heated to 550 co become opalescent. Ir spectra related to water and OH groups are changed. Photoluminescence intensity of all three bands, UV (250 nm), yellow (600 nm) and red (740 nm) strongly diminished and disappeared after heating to 660 C°. Radiation induced red luminescence of non-bridging oxygen luminescence center (NBO) appeared in crystal after heat treatment. We had observed a crystalline version of this center (l. Skuja et al, Nuclear Instruments and Methods in Physics Research B 286,159-168 (2012)). Effect of heat treatment explained as sedimentation of phosphorus in some state. Keeping of treated sample at 450-500 C° leads to partial revival of ability to create yellow luminescence center under irradiation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Experimental determination of band offsets of NiO-based thin film heterojunctions

The energy band diagrams of NiO-based solar cell structures that use various n-type oxide semiconductors such as ZnO, Mg{sub 0.3}Zn{sub 0.7}O, Zn{sub 0.5}Sn{sub 0.5}O, In{sub 2}O{sub 3}:Sn (ITO), SnO{sub 2}, and TiO{sub 2} were evaluated by photoelectron yield spectroscopy. The valence band discontinuities were estimated to be 1.6 eV for ZnO/NiO and Mg{sub 0.3}Zn{sub 0.7}O/NiO, 1.7 eV for Zn{sub 0.5}Sn{sub 0.5}O/NiO and ITO/NiO, and 1.8 eV for SnO{sub 2}/NiO and TiO{sub 2}/NiO heterojunctions. By using the valence band discontinuity values and corresponding energy bandgaps of the layers, energy band diagrams were developed. Judging from the band diagram, an appropriate solar cell consisting of p-type NiO and n-type ZnO layers was deposited on ITO, and a slight but noticeable photovoltaic effect was obtained with an open circuit voltage (V{sub oc}) of 0.96 V, short circuit current density (J{sub sc}) of 2.2 μA/cm{sup 2}, and fill factor of 0.44.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Fe ion-implanted TiO{sub 2} thin film for efficient visible-light photocatalysis

This work shows the application of metal ion-implantation to realize an efficient second-generation TiO{sub 2} photocatalyst. High fluence Fe{sup +} ions were implanted into thin TiO{sub 2} films and subsequently annealed up to 550 °C. The ion-implantation process modified the TiO{sub 2} pure film, locally lowering its band-gap energy from 3.2 eV to 1.6–1.9 eV, making the material sensitive to visible light. The measured optical band-gap of 1.6–1.9 eV was associated with the presence of effective energy levels in the energy band structure of the titanium dioxide, due to implantation-induced defects. An accurate structural characterization was performed by Rutherford backscattering spectrometry, transmission electron microscopy, Raman spectroscopy, X-ray diffraction, and UV/VIS spectroscopy. The synthesized materials revealed a remarkable photocatalytic efficiency in the degradation of organic compounds in water under visible light irradiation, without the help of any thermal treatments. The photocatalytic activity has been correlated with the amount of defects induced by the ion-implantation process, clarifying the operative physical mechanism. These results can be fruitfully applied for environmental applications of TiO{sub 2}.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Optical properties of large-area ultrathin MoS{sub 2} films: Evolution from a single layer to multilayers

We investigated the optical properties of ultrathin MoS{sub 2} films (number of layers: N = 1, 2, 4, and 12) using Raman spectroscopy, photoluminescence (PL) spectroscopy, and spectroscopic ellipsometry. We estimated the layer thicknesses based on Raman spectra. We characterized the microstructural properties of a single-layer MoS{sub 2} film using atomic force microscopy. We measured the lowest-energy A and B excitons using PL spectroscopy. We measured the ellipsometric angles (Ψ and Δ) of MoS{sub 2} thin films using spectroscopic ellipsometry, and obtained the dielectric functions as the films' thickness changed from a single layer to multi-layers. We determined the films' optical gap energies from the absorption coefficients. Applying the standard critical point model to the second derivative of the dielectric function (d{sup 2}ε(E)/dE{sup 2}), we determined several critical point energies. The d{sup 2}ε(E)/dE{sup 2} spectra showed doublet peaks around 3 eV corresponding to the C and D transitions, as well as doublet peaks around 2 eV corresponding to the A and B transitions. These doublet structures at 3 eV are attributed to the transitions in the Brillouin zone between the Γ and K points.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Theoretical investigation of stabilities and optical properties of Si{sub 12}C{sub 12} clusters

By sorting through hundreds of globally stable Si{sub 12}C{sub 12} isomers using a potential surface search and using simulated annealing, we have identified low-energy structures. Unlike isomers knit together by Si–C bonds, the lowest energy isomers have segregated carbon and silicon regions that maximize stronger C–C bonding. Positing that charge separation between the carbon and silicon regions would produce interesting optical absorption in these cluster molecules, we used time-dependent density functional theory to compare the calculated optical properties of four isomers representing structural classes having different types of silicon and carbon segregation regions. Absorptions involving charge transfer between segregated carbon and silicon regions produce lower excitation energies than do structures having alternating Si–C bonding for which frontier orbital charge transfer is exclusively from separated carbon atoms to silicon atoms. The most stable Si{sub 12}C{sub 12} isomer at temperatures below 1100 K is unique as regards its high symmetry and large optical oscillator strength in the visible blue. Its high-energy and low-energy visible transitions (1.15 eV and 2.56 eV) are nearly pure one-electron silicon-to-carbon transitions, while an intermediate energy transition (1.28 eV) is a nearly pure carbon-to-silicon one-electron charge transfer.

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

A guided-ion beam study of the collisions and reactions of I{sup +} and I{sub 2}{sup +} with I{sub 2}

Growing interest in developing and testing iodine Hall effect thrusters requires measurements of the cross sections of reactions that generate low energy plasma following discharge. Limited experimental and theoretical work necessitates a decisive experiment to elucidate the charge exchange and collision-induced dissociation channels. To this end, we have used guided-ion beam techniques to measure cross sections for both I{sup +} + I{sub 2} and I{sub 2}{sup +}+I{sub 2} collisions. We present total collision cross sections as well as collision-induced dissociation cross sections for center-of-mass collision energies ranging from 0.5 to 200 eV for molecular iodine cations. Similarly, we present total collision cross section and charge-exchange cross sections for atomic iodine cations for center-of-mass collision energies ranging from 0.67 to 167 eV. Time-of-flight measurements of the collision products allow determination of velocity distributions, which show evidence of complex formation of I{sub 3}{sup +} from the I{sup +} + I{sub 2} reaction at collision energies below 6 eV.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Interplay of electron correlations and localization in disordered β-tantalum films: Evidence from dc transport and spectroscopic ellipsometry study

We report the dc transport (5 K ≲ T ≲ 380 K) and spectroscopic ellipsometry (0.8 eV ≤ hν ≤ 8.5 eV, T ≃ 300 K) study of β-Ta films prepared by rf sputtering deposition as a function of their thickness in the range 2.5 nm ≲ d ≲ 200 nm. The dc transport of the β-Ta films with a thickness d ≳ 25 nm is characterized by negative temperature coefficient of resistivity (TCR) caused by localization effects peculiar of highly disordered metals. Their dielectric function spectra display non-metallic-like behavior due to the presence of the pronounced band at 2 eV. We found that with increasing TCR absolute value, specifying elevated degree disorder, the optical spectral weight (SW) of free charge carriers decreases. The associated SW is recovered in the range of Mott-Hubbard transitions, indicating the mechanism of localization enhancement by electronic correlations in disordered metals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Identification of titanium-hydrogen complexes with up to four hydrogen atoms in silicon

Using high-resolution Laplace deep level transient spectroscopy studies, several TiH-related complexes (E40′, E170, E170′, and E260) were observed in wet-chemically etched and H-plasma treated n-type Si. We assign E40′ and E170 to two different configurations of Ti with one H atom. Both of them are shown to behave as single donor states with an activation enthalpy of E{sub C} − 0.07 eV (E40′) and E{sub C} − 0.34 eV (E170) in the upper half of Si. E170′ with an activation energy of E{sub C} − 0.37 eV is correlated with the donor state of the Ti{sub i}H{sub 2} defect, whereas E260 is attributed to the donor state of Ti{sub i}H{sub 3}. Besides the TiH defects, the presence of electrically inactive TiH{sub 4} is reported. No titanium-hydrogen-related levels were observed in p-type Si.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Effect of interfacial coupling on photocatalytic performance of large scale MoS{sub 2}/TiO{sub 2} hetero-thin films

Interface electronic behavior of two-dimensional large scale MoS{sub 2}/TiO{sub 2} hetero-thin films has been studied using photoemission spectroscopy. We show a clear experimental evidence for type II band alignment and upward band bending (∼0.55 eV) at the interface of this system. The valence band offset at monolayer MoS{sub 2}/TiO{sub 2} interface was measured to be 2.15 eV, while the conduction band offset was 1.00 eV. The unique interface band positions introduce a strong build-in electric field for efficient electron-hole separation. In addition, thermal treatment results in better interfacial coupling and charge separation efficiency thus enhanced visible light photoactivity. Our results explicate the mechanism and emphasize its huge potential in visible light photocatalysis.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Barrierless proton transfer across weak CH⋯O hydrogen bonds in dimethyl ether dimer

We present a combined computational and threshold photoelectron photoion coincidence study of two isotopologues of dimethyl ether, (DME − h{sub 6}){sub n} and (DME − d{sub 6}){sub n}n = 1 and 2, in the 9–14 eV photon energy range. Multiple isomers of neutral dimethyl ether dimer were considered, all of which may be present, and exhibited varying C–H⋯O interactions. Results from electronic structure calculations predict that all of them undergo barrierless proton transfer upon photoionization to the ground electronic state of the cation. In fact, all neutral isomers were found to relax to the same radical cation structure. The lowest energy dissociative photoionization channel of the dimer leads to CH{sub 3}OHCH{sub 3}{sup +} by the loss of CH{sub 2}OCH{sub 3} with a 0 K appearance energy of 9.71 ± 0.03 eV and 9.73 ± 0.03 eV for (DME − h{sub 6}){sub 2} and deuterated (DME − d{sub 6}){sub 2}, respectively. The ground state threshold photoelectron spectrum band of the dimethyl ether dimer is broad and exhibits no vibrational structure. Dimerization results in a 350 meV decrease of the valence band appearance energy, a 140 meV decrease of the band maximum, thus an almost twofold increase in the ground state band width, compared with DME − d{sub 6} monomer.

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

Effect of secondary electron emission on the plasma sheath

In this experiment, plasma sheath potential profiles are measured over boron nitride walls in argon plasma and the effect of secondary electron emission is observed. Results are compared to a kinetic model. Plasmas are generated with a number density of 3 × 10{sup 12} m{sup −3} at a pressure of 10{sup −4} Torr-Ar, with a 1%–16% fraction of energetic primary electrons. The sheath potential profile at the surface of each sample is measured with emissive probes. The electron number densities and temperatures are measured in the bulk plasma with a planar Langmuir probe. The plasma is non-Maxwellian, with isotropic and directed energetic electron populations from 50 to 200 eV and hot and cold Maxwellian populations from 3.6 to 6.4 eV and 0.3 to 1.3 eV, respectively. Plasma Debye lengths range from 4 to 7 mm and the ion-neutral mean free path is 0.8 m. Sheath thicknesses range from 20 to 50 mm, with the smaller thickness occurring near the critical secondary electron emission yield of the wall material. Measured floating potentials are within 16% of model predictions. Measured sheath potential profiles agree with model predictions within 5 V (∼1 T{sub e}), and in four out of six cases deviate less than the measurement uncertainty of 1 V.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Spectroscopic and electrical calculation of band alignment between atomic layer deposited SiO{sub 2} and β-Ga{sub 2}O{sub 3} (2{sup ¯}01)

The energy band alignment between atomic layer deposited (ALD) SiO{sub 2} and β-Ga{sub 2}O{sub 3} (2{sup ¯}01) is calculated using x-ray photoelectron spectroscopy and electrical measurement of metal-oxide semiconductor capacitor structures. The valence band offset between SiO{sub 2} and Ga{sub 2}O{sub 3} is found to be 0.43 eV. The bandgap of ALD SiO{sub 2} was determined to be 8.6 eV, which gives a large conduction band offset of 3.63 eV between SiO{sub 2} and Ga{sub 2}O{sub 3}. The large conduction band offset makes SiO{sub 2} an attractive gate dielectric for power devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Interpretation of the vacuum ultraviolet photoabsorption spectrum of iodobenzene by ab initio computations

Identification of many Rydberg states in iodobenzene, especially from the first and fourth ionization energies (IE{sub 1} and IE{sub 4}, X{sup 2}B{sub 1} and C{sup 2}B{sub 1}), has become possible using a new ultraviolet (UV) and vacuum-ultraviolet (VUV) absorption spectrum, in the region 29 000-87 000 cm{sup −1} (3.60-10.79 eV), measured at room temperature with synchrotron radiation. A few Rydberg states based on IE{sub 2} (A{sup 2}A{sub 2}) were found, but those based on IE{sub 3} (B{sup 2}B{sub 2}) are undetectable. The almost complete absence of observable Rydberg states relating to IE{sub 2} and IE{sub 3} (A{sup 2}A{sub 2} and B{sup 2}B{sub 2}, respectively) is attributed to them being coupled to the near-continuum, high-energy region of Rydberg series converging on IE{sub 1}. Theoretical studies of the UV and VUV spectra used both time-dependent density functional (TDDFT) and multi-reference multi-root doubles and singles-configuration interaction methods. The theoretical adiabatic excitation energies, and their corresponding vibrational profiles, gave a satisfactory interpretation of the experimental results. The calculations indicate that the UV onset contains both 1{sup 1}B{sub 1} and 1{sup 1}B{sub 2} states with very low oscillator strength, while the 2{sup 1}B{sub 1} state was found to lie under the lowest ππ{sup ∗} 1{sup 1}A{sub 1} state. All three of these {sup 1}B{sub 1} and {sup 1}B{sub 2} states are excitations into low-lying σ{sup ∗} orbitals. The strongest VUV band near 7 eV contains two very strong ππ{sup ∗} valence states, together with other weak contributors. The lowest Rydberg 4b{sub 1}6s state (3{sup 1}B{sub 1}) is very evident as a sharp multiplet near 6 eV; its position and vibrational structure are well reproduced by the TDDFT results.

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

Electronic band structure of compressively strained Ge{sub 1−x}Sn{sub x} with x < 0.11 studied by contactless electroreflectance

Contactless electroreflectance is applied to study direct optical transitions from the heavy hole, light hole, and spin-orbit split-off band to the conduction band in compressively strained Ge{sub 1−x}Sn{sub x} layers of various Sn concentrations at room temperature. It is shown that the energies of these transitions are in very good agreement with theoretical predictions, which take into account non-linear variation of bandgap and spin-orbit splitting plus the strain-related shifts obtained from the Bir-Pikus theory. The bowing parameter for the direct bandgap has been determined to be 1.8 ± 0.2 eV and agree with this one obtained within ab initio calculations, which is 1.97 eV (for indirect bandgap the bowing parameter is 0.26 eV)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Magnetized retarding field energy analyzer measuring the particle flux and ion energy distribution of both positive and negative ions

This paper presents the development of a magnetized retarding field energy analyzer (MRFEA) used for positive and negative ion analysis. The two-stage analyzer combines a magnetic electron barrier and an electrostatic ion energy barrier allowing both positive and negative ions to be analyzed without the influence of electrons (co-extracted or created downstream). An optimal design of the MRFEA for ion-ion beams has been achieved by a comparative study of three different MRFEA configurations, and from this, scaling laws of an optimal magnetic field strength and topology have been deduced. The optimal design consists of a uniform magnetic field barrier created in a rectangular channel and an electrostatic barrier consisting of a single grid and a collector placed behind the magnetic field. The magnetic barrier alone provides an electron suppression ratio inside the analyzer of up to 6000, while keeping the ion energy resolution below 5 eV. The effective ion transparency combining the magnetic and electrostatic sections of the MRFEA is measured as a function of the ion energy. It is found that the ion transparency of the magnetic barrier increases almost linearly with increasing ion energy in the low-energy range (below 200 eV) and saturates at high ion energies. The ion transparency of the electrostatic section is almost constant and close to the optical transparency of the entrance grid. We show here that the MRFEA can provide both accurate ion flux and ion energy distribution measurements in various experimental setups with ion beams or plasmas run at low pressure and with ion energies above 10 eV.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Threshold photoelectron spectroscopy of unstable N-containing compounds: Resolution of ΔK subbands in HNCO{sup +} and vibrational resolution in NCO{sup +}

The threshold photoelectron spectra (TPES) of two unstable nitrogen-containing species, HNCO and NCO, were recorded utilizing vacuum ultraviolet synchrotron radiation. Both are intermediates in combustion processes and play a role in the removal of nitrogen oxides from exhaust gases. The rovibronic structure of the first band in the TPES of HNCO{sup +} was analyzed within the framework of an orbital ionization model, and the resolved structure of the origin band was assigned to ΔK subbands. An ionization energy of 11.602 ± 0.005 eV was determined and the vibrational structure of the cationic ground state was analyzed by a Franck-Condon fit. Low lying electronically excited states of HNCO{sup +} were also observed. In a second series of experiments, the NCO radical was generated by flash pyrolysis from chlorine isocyanate. The ionization energy to the X{sup +} {sup 3}Σ{sup −} ground state was determined to be 11.76 ± 0.02 eV, while for the a{sup +1}Δ state, a value of 12.93 ± 0.02 eV was obtained. Vibrational structure was observed for both states, and bands were assigned by Franck-Condon simulations.

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

Photoelectric properties of the metamorphic InAs/InGaAs quantum dot structure at room temperature

We present the study of optical and photoelectric properties of InAs quantum dots (QDs) grown on a metamorphic In{sub 0.15}Ga{sub 0.85}As buffer layer: such nanostructures show efficient light emission in the telecom window at 1.3 μm (0.95 eV) at room temperature. We prepared a sample with vertical geometry of contacts isolated from the GaAs substrate. The structure is found to be photosensitive in the spectral range above 0.9 eV at room temperature, showing distinctive features in the photovoltage and photocurrent spectra attributed to QDs, InAs wetting layer, and In{sub 0.15}Ga{sub 0.85}As metamorphic buffer, while a drop in the photoelectric signal above 1.36 eV is related to the GaAs layer. No effect of defect centers on the photoelectrical properties is found, although they are observed in the absorption spectrum. We conclude that metamorphic QDs have a low amount of interface-related defects close to the optically active region and charge carriers can be effectively collected into InAs QDs.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Wavelength dependence of femtosecond laser-induced damage threshold of optical materials

An experimental and numerical study of the laser-induced damage of the surface of optical material in the femtosecond regime is presented. The objective of this work is to investigate the different processes involved as a function of the ratio of photon to bandgap energies and compare the results to models based on nonlinear ionization processes. Experimentally, the laser-induced damage threshold of optical materials has been studied in a range of wavelengths from 1030 nm (1.2 eV) to 310 nm (4 eV) with pulse durations of 100 fs with the use of an optical parametric amplifier system. Semi-conductors and dielectrics materials, in bulk or thin film forms, in a range of bandgap from 1 to 10 eV have been tested in order to investigate the scaling of the femtosecond laser damage threshold with the bandgap and photon energy. A model based on the Keldysh photo-ionization theory and the description of impact ionization by a multiple-rate-equation system is used to explain the dependence of laser-breakdown with the photon energy. The calculated damage fluence threshold is found to be consistent with experimental results. From these results, the relative importance of the ionization processes can be derived depending on material properties and irradiation conditions. Moreover, the observed damage morphologies can be described within the framework of the model by taking into account the dynamics of energy deposition with one dimensional propagation simulations in the excited material and thermodynamical considerations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗