bluuebunny/crossref_metadata_2025 huggingface dataset:
Manifestation of structural defects in photoluminescence from GaN
Authors: Reshchikov M. A. , Jasinski J. , Yun F. , He L. , Liliental-Weber Z. , Morkoç H.
Published: April 2011

Sharp peaks of unidentified nature are detected in the low-temperature photoluminescence (PL) spectrum of undoped GaN in the photon energy range between 3.0 and 3.46 eV. These PL lines are commonly attributed to excitons bound to yet unidentified structural defects. We analyzed X-ray diffraction data in a large set of GaN samples grown by molecular beam epitaxy in order to find any correlation between these unusual PL peaks and the GaN crystal structure. Moreover, in selected samples exhibiting such peaks, cross-sectional transmission electron microscopy was taken in an effort to detect the presence and density of various structural defects. The preliminarily results indicate that most of unusual PL lines in GaN (Y lines) are not directly related to the observed structural defects, such as edge, screw, mixed dislocations, or stacking faults. However, there exists the possibility that point defects trapped at dislocations or other structural defects are responsible for these PL lines.
Strong dependence of the fundamental band gap on the alloy composition in cubic InxGa1-xN and InxAl1-xN alloys
Authors: Dridi Z. , Bouhafs B. , Ruterana P.
Published: April 2011

Ab initio total energy calculations, based on the full-potential augmented plane wave method within the local density approximation, are used to study the influence of alloying on the structural and electronic properties of cubic InxGa1-xN and InxAl1-xN ordered alloys in the chalcopyrite and luzonite structures.

We have investigated the lattice parameters and band gap energies. The lattice parameters, exhibit an upward bowing of -0.05 Å and -0.15 Å, respectively for the InxGa1-xN and InxAl1-xN alloys. The composition dependence of the band gap shows a bowing parameter of 1.36 eV for InxGa1-xN. The band gap InxAl1-xN alloys exhibits a bowing parameter of 3.19 eV, and a direct to indirect band gap transition at x (Al) = 0.83. The large bowing effect in InxAl1-xN alloys has been discussed in terms of a composition dependent bowing parameter.
Luminescent Holmium Doped Amorphous AlN Thin Films for use as Waveguides and Laser Cavities.
Authors: Maqbool Muhammad , Richardson H. H. , Van Patten P. G. , Kordesch M. E.
Published: April 2011

Holmium doped AlN thin films have been deposited on flat silicon substrates and optical fibers of different diameters, in order to fabricate resonant cavities for lasers. In particular, the films on fiber substrates are expected to perform as cylindrical waveguides or “whispering gallery” mode cavities. The AlN films are deposited by reactive sputtering at liquid nitrogen temperature, using 100–200 Watts RF power, 5–8 mTorr nitrogen, using a metal target of Al and Ho. One micron thick films of AlN:Ho have been deposited on flat silicon substrates and 10 micron thick films on 80μm and smaller optical fibers. X-ray Diffraction and SEM studies show that films deposited on flat silicon are amorphous while those deposited on the fibers show columnar growth and some gain structure, most probably due to a temperature rise at the substrate during deposition. Cathodoluminescence (CL) emission is observed in thermally activated AlN:Ho in both crystalline and amorphous forms. An intense, narrow emission peak is observed at 549 nm. Less intense peaks are also observed at 362nm, 394 nm, 461 nm, and 659 nm. The most promising wavelength for our design is emission from the 5S2 → 5I8 transition at 549 nm.
Time-Resolved Reflectivity Studies of Electric Field Effects in III-Nitride Semiconductors
Authors: Wraback M. , Shen H. , Sampath A. V. , Collins C. J. , Garrett G. A. , Sarney W. L. , Fedyunin Y. , Cabalu J. , Moustakas T. D.
Published: April 2011

Comparison of room temperature, frequency degenerate and nondegenerate femtosecond pump-probe reflectivity measurements reveal an additional component in the transient reflectivity (ΔR) decays for near bandgap probe in GaN that is attributed to a reduction of bandedge broadening due to screening of internal electric fields by photogenerated carriers. Pump-probe measurements with pulses spectrally centered at 363 nm show a ΔR component with a nanosecond decay time that is essentially absent from data obtained from nondegenerate experiments performed under identical pumping conditions, but with a probe at 385 nm far enough from the bandedges to minimize the electric field contribution and monitor primarily the carrier lifetime in the band states. The observation of significantly shorter decay times for 385 nm probe suggests that the carriers leave the band states on a picosecond time scale, but the slow decay of the field screening observed with 363 nm probe implies that they recombine from trap states at longer times. In addition, this field screening provides a mechanism for generation and detection of strain pulses in strained GaN and AlGaN epilayers that is used to measure the longitudinal sound velocity in bulk GaN and AlGaN with up to 0.4 Al content.
High Temperature Operation of A New Normally-Off AlGaN/GaN HFET on Si Substrate
Authors: Yoshida Seikoh , Li Jiang , Wada Takahiro , Takehara Hironari
Published: April 2011

We report on the novel normally-off AlGaN/p-type GaN heterojunction field effect transistors (HFETs). We grew the AlGaN/p-GaN heterostructure on p-type Si (111) substrate using a metalorganic chemical vapor deposition (MOCVD). A homogeneous buffer layer was first formed on a Si (111) substrate at 1123 K. After that, AlGaN (30 nm)/high resistive p-type GaN (500 nm) heterostructure was also grown at 1303 K without cracking. We fabricated an HFET using AlGaN/p-type GaN on Si substrate. The ohmic electrode material was Al/Ti/Au and the gate electrode was Pt/Au. The distance between the source and the drain was 0.01 mm. The gate length and width were 2000 nm and 0.15 mm, respectively. As a result, the HFET was operated at the condition of the positive gate bias. The pinch-off voltage was 0 V. A normally-off operation was thus confirmed for the first time. The breakdown voltage of FET was over 250 V. We also confirmed that the normally-off HFET was operated at 573 K for over 150 h.
High-Power Characteristics of GaN/InGaN Double Heterojunction Bipolar Transistors with a Regrown p-InGaN Base Layer
Authors: Makimoto Toshiki , Yamauchi Yoshiharu , Kumakura Kazuhide
Published: April 2011

We have investigated high-power characteristics of GaN/InGaN double heterojunction bipolar transistors on SiC substrates grown by metalorganic vapor phase epitaxy. The p-InGaN extrinsic base layers were regrown to improve ohmic characteristics of the base. Base-collector diodes showed low leakage current at their reverse bias voltages due to a wide bandgap of a GaN collector, resulting in a high-voltage transistor operation. A 90 μm × 50 μm device operated up to a collector-emitter voltage of 28 V and a collector current of 0.37 A in its common-emitter current-voltage characteristics at room temperature, which corresponds to a DC power of 10.4 W. A collector current density and a power density are as high as 8.2 kA/cm2 and 230 kW/cm2, respectively. These results show that nitride HBTs are promising for high-power electronic devices.
Electrical and Optical Properties of Carbon Doped Cubic GaN Epilayers Grown Under Extreme Ga Excess
Authors: As D. J. , Pacheco-Salazar D. G. , Potthast S. , Lischka K.
Published: April 2011

P-type doping of cubic GaN by carbon is reported with maximum hole concentration of 2 6.1×1018cm-3and hole mobility of 23.5 cm /Vs at room temperature, respectively. The cubic GaN:C was grown by rf-plasma assisted molecular beam epitaxy (MBE) under Ga-rich growth conditions on a semiinsulating GaAs (001) substrate (3 inches wafer). E-beam evaporation of a graphite rode with an C-flux of 1×1012cm-2s-1was used for C-doping of the c-GaN. Optical microscopy, Hall-effect measurements and photoluminescence were performed to investigate the morphological, electrical and optical properties of cubic GaN:C. Under Ga-rich growth conditions most part of the carbon atoms were incorporated substitutially on N-site giving p-type conductivity. Our results verify that effective p-type doping of c-GaN can be achieved under extrem Ga excess.
Optical Properties of Mn-doped GaN
Authors: Gelhausen O. , Malguth E. , Phillips M. R. , Goldys E. M. , Strassburg M. , Hoffmann A. , Graf T. , Gjukic M. , Stutzmann M.
Published: April 2011

Molecular beam epitaxy-grown GaN with different Mn concentrations (5–23 × 10 cm-3) and codoped with Si were investigated by cathodoluminescence (CL) spectroscopy and optical transmission measurements. In the GaN:Mn, an intense absorption peak at 1.414 +/- 0.002 eV was observed. This peak was attributed to an internal T2∼> E transition of the deep neutral Mn3+ state since its intensity scaled with the Mn3+ concentration. The CL measurements showed that Mn-doping concentrations around 1020 cm-3 had three effects on the emission spectrum: (i) the donor bound exciton at 3.460 eV was reduced by more than one order of magnitude, (ii) the donor-acceptor-pair band at 3.27 eV was completely quenched and (iii) the yellow luminescence centered at 2.2 eV was the strongly decreased. The latter two effects were attributed to a reduced concentration of VGa. In the infrared spectral range, three broad, Mn-doping related CL emission bands centered at 1.01 ± 0.02 eV, 1.09 ± 0.02 eV and 1.25 ± 0.03 eV were observed. These bands might be related to deep donor complexes, which are generated as a result of the heavy Mn-doping, rather than internal transitions at the Mn atom.
MBE Growth Study of GaAsSbN/GaAs Single Quantum Wells
Authors: Wu Liangjin , Iyer Shanthi , Nunna Kalyan , Li Jia , Bharatan Sudhakar , Collis Ward , Matney Kevin
Published: April 2011

In this work, the growth and properties of GaAsSbN single quantum wells are investigated. The heterostructures were grown on GaAs substrates in an elemental solid source molecular beam epitaxy system with a RF plasma nitrogen source. A systematic study has been carried out to determine the influence of growth temperature on the optical properties of the layers. For reference low temperature photoluminescence (PL) characteristics of the GaAsSb/GaAs QWs as a function of Sb is also presented. A significant increase in PL intensity with a corresponding blue shift in emission energy and a decrease in full width at half maximum (FWHM) has been observed on annealing the GaAsSbN/GaAs sample in a nitrogen ambient at 700°C. PL emission wavelength as long as 1.52 μm at room temperature has been obtained on annealed samples.
Modeling of recombination lifetimes in charge-separation device structures
Authors: Dashdorj Jamiyanaa , Ahrenkiel Richard , Metzger Wyatt
Published: April 2011

Recombination processes in semiconductors are inhibited by charge-separation mechanisms that are produced in structures such as pn junctions. In this work, we calculated the recombination rates in Si and GaAs n+p junction structures by both analytical and numerical techniques. Transport effects can be divided into three physical phenomena. The first is the flow of carriers that results in the build up of a junction voltage or charge-separation. The time constant of this effect is determined by the junction dopant concentrations and the mobilities. The second is the flow of carriers due to diffusion. The time constant of this event is a function of the carrier diffusion coefficient and physical width of the absorbing medium. In most semiconductor structures, this time constant is relatively short relative to the recombination lifetime. The third effect is the discharge of the open-circuit voltage of the junction by minority carrier charge injection. A heuristic analysis of the long-term decay in this regime indicates that the decay time varies as CkT /qJ0, where C is the junction capacitance and J0 is the dark current. In this regime, the decay time is a function of recombination lifetime through the dependence of J0, but the measured excess carrier decay time is often not the true recombination lifetime of the underlying materials.
Defects and Surfactant Action of Antimony on GaAs and GaAs1-xNx on GaAs [100] by Molecular Beam Epitaxy
Authors: Cheah W. K. , Fan W. J. , Yoon S. F. , Wicaksono S. , Liu R. , Wee A. T. S.
Published: April 2011

Low temperature (4.5K) photoluminescence (PL) measurements of GaAs(N):Sb on GaAs grown by solid source molecular beam epitaxy (MBE) show a Sb-related defect peak at ∼1017nm (1.22eV). The magnitude of the Sb-related impurity PL peak corresponds in intensity with the prominence of the additional two-dimensional [115] high-resolution x-ray diffraction (HRXRD) defect peaks. The elimination of these defects can be a measure of the improvement in crystal quality of GaAsN:Sb and a Sb flux ≥ 1.3×10−8 Torr is needed to invoke the surfactant behavior in III-V dilute nitride MBE growth for a growth rate of 1μm/hr.
Fabrication of side-illuminated p-i-n Photodiode with waveguide layers
Authors: Jeon Byungok , Yang Seungkee , Kang Hwayoung , Rhee Doyoung
Published: April 2011

We present the fabrication of side-illuminated p-i-n photodiode. This is suitable for passive integration on the planar lightwave circuit (PLC), which is able to make the low-cost packages. Epitaxial structures have two main layers, diode layer and waveguide layer. It was determined by beam propagation method (BPM) simulation. To get a high responsivity and reliability, each layer must be etched. The alignment tolerances were tested. We carried out some experiments on various methods and several devices were satisfactory for the optical communication system at 1.3- and 1.55 micrometer wavelengths.
Determination of the Nitrogen Acceptor Ionization Energy in Zinc Oxide by Photoluminescence Spectroscopy
Authors: Wang Lijun , Garces N. Y. , Halliburton L. E. , Giles N. C.
Published: April 2011

Photoluminescence (PL) experiments performed on bulk ZnO crystals are used to establish the ionization energy of the substitutional nitrogen acceptor. The temperature dependence of the nitrogen-related electron-acceptor (e,A°) emission band has been monitored in as-grown single crystals. A lineshape analysis of this band is used to determine the acceptor ionization energy. The temperature variation of the ZnO band gap was included in our analysis and the low-temperature acceptor ionization energy for substitutional nitrogen at an oxygen site in ZnO was found to be EA = 209 ± 3 meV. Electron paramagnetic resonance and Hall-effect measurements were also used to characterize these bulk ZnO samples.
Near-Field Magneto-Photoluminescence of Singe Self-Organized Quantum Dots.
Authors: Mintairov A. M. , Vlasov A. S. , Merz J. L.
Published: April 2011

We present results obtained using low temperature near-field scanning optical microscopy for the measurements of Zeeman splitting and the diamagnetic shift of single self-organized InAs/AlAs, InAs/GaAs and InP/GaInP quantum dots. The measurements allow us to relate the bimodal size distribution of InAs dots with variations in In content. For single InP QDs we observed a strong circular polarization at zero magnetic field accompanied with a negative energy shift, suggesting that strong internal magnetic fields exist in these QDs.
Quantum Dot Lasers and Amplifiers
Authors: Pohl Udo W. , Bimberg Dieter
Published: April 2011

Progress in self-organized epitaxy of quantum dots lead to fabrication of edge- and surface-emitting lasers featuring excellent device characteristics. Exceedingly low lasing threshold, an internal quantum efficiency near unity, high characteristic temperature was proved, and high power operation was demonstrated. We present recent achievements on lasers and optical amplifiers based on InGaAs quantum dots in GaAs matrix.
Nanoengineered Quantum Dot Active Medium for Thermally-Stable Laser Diodes
Authors: Tokranov V. , Yakimov M. , Katsnelson A. , Lamberti M. , Agnello G. , Oktyabrsky S.
Published: April 2011

The influence of a nanoengineering procedure on the properties of single- and multi-layer self-assembled InAs quantum dot (QDs) has been studied using photoluminescence, transmission electron microscopy (TEM), and electroluminescence. Optical properties of QDs were optimized by shape engineering via adjustment of a GaAs overlayer thickness prior to a heating (truncation) step. TEM micrographs have confirmed that the employed growth procedure results in truncated pyramidal QDs covered entirely by AlAs with smooth top interfaces. Truncated QDs with two-monolayer-thick AlAs capping have demonstrated a strong blue shift of ground state (GS) energies and up to 10 meV larger separation between GS and first excited state energy levels as compared to non-capped QDs. We believe that AlAs capping, in combination with truncation procedure, results in significant suppression of carrier transport between QDs within the same layer as well as between QD layers. A record high characteristic temperature for GS lasing threshold, T0 = 380 K up to 55 °C, as well as a maximum saturated gain of 16 (5.3 per layer) cm-1, were measured for a 1.22 μm edge-emitting lasers with this truncated triple layer QD gain medium. A maximum saturated gain, 27 (3.9 per layer) cm-1, and T0 = 110 K have been demonstrated in a 1.19 μm lasers with truncated seven layer QD medium.
First electrically injected QD-MCLED emitting at 1.3 μm, grown by metal organic chemical vapour deposition
Authors: Tasco V. , Todaro M. T. , De Giorgi M. , De Vittorio M. , Cingolani R. , Passaseo A.
Published: April 2011

We present the fabrication of a Quantum Dot Microcavity Light Emitting Diode (QD-MCLED) emitting at 1.3 μm at room temperature. The long wavelength emission is achieved by using for the first time InGaAs QDs directly grown on GaAs, by metal organic chemical vapour deposition. Electroluminescence bright emission, peaked at 1298 nm with a FWHM of 6.5 meV and a line shape independent on the applied bias is found.
Growth and characterization of InAs quantum dots on GaAs (100) emitting at 1.31μm.
Authors: Celibert V. , Salem B. , Guillot G. , Bru-Chevallier C. , Grenouillet L. , Duvaut P. , Gilet P. , Million A.
Published: April 2011

Self-organized InAs quantum dots (QDs) were grown in the Stranski-Krastanov regime, by gas-source molecular beam epitaxy (GSMBE), on (100) GaAs substrates. Two important parameters have been optimized in order to grow high quality QDs with a very good reproducibility: InAs growth rate and GaAs cap layer deposition rate. Atomic force microscopy (AFM) analysis shows a unimodal QD distribution and the room temperature photoluminescence (RTPL) spectrum of the optimized sample reveals a 1.3 μm emission with a 19 meV full width at half maximum (FWHM). Photoluminescence (PL) measurements versus excitation power density and photoluminescence excitation (PLE) measurements clearly show multi-component PL emission from transitions associated with fundamental and related excited states of QDs. Furthermore a good growth reproducibility is observed. The results are promising for further work which will lead to laser fabrication.
Optical Properties of Bulk and Epitaxial ZnO for Waveguide Applications
Authors: Ganesan S. , Feng Z. C. , Mehta D. , Kandoor S. , Wornyo E. J. , Nause J. , Ferguson I.
Published: April 2011

This paper investigates the optical properties of bulk and epitaxial ZnO layers. High quality undoped and doped bulk ZnO crystals have been produced by melt growth techniques in addition to ZnO thin films grown by Metalorganic Chemical Vapor Deposition (MOCVD) on silicon, sapphire and the bulk ZnO substrates. This work focuses on investigating the suitability of bulk and epitaxial ZnO for waveguide applications using various spectroscopic techniques. The photoluminescence showed the dominance of strong and narrow band due to the band edge emissions for undoped ZnO. Ultraviolet-visible transmission data revealed the variation of the bandgap with different doping elements. Raman spectra showed a narrow and strong peak, corresponding to the E2 mode at 438 cm-1, characteristic of the ZnO crystallinity. A broad 2LO peak appeared near 1150 cm-1 due to the coupling between LO phonons and free carriers. A clear variation in refractive index with doping was observed by spectroscopic ellipsometery suggesting that ZnO could be used for waveguide applications.
Spin-Dependent Optical Processes in II-VI Diluted Magnetic Semiconductor Nanostructures
Authors: Oka Yasuo , Kayanuma Kentaro , Sakuma Mio , Uetake Ayahito , Souma Izuru , Chen Zhanghai , Murayama Akihiro
Published: April 2011

Nanostructures of diluted magnetic semiconductors have potential possibility for the spin electronics application due to their large magneto-optical effects and the quantum confinement effects for band electrons. We have fabricated quantum wells and quantum dots by II-VI diluted magnetic semiconductors of Cd1-xMnxTe, Zn1-xMnxTe and Zn1-x-yCdyMnxSe. These nanostructures showed spin injection, spin switching and ultrafast spin relaxation processes derived in the confined nanostructures. Hybrid structures consisting of diluted magnetic semiconductor and Co showed Mn spin polarizations induced by the Co wire shape.
Mixing Rocksalt and Wurtzite Structure Binary Nitrides to Form Novel Ternary Alloys: ScGaN and MnGaN
Authors: Constantin Costel , Al-Brithen Hamad , Haider Muhammad B. , Ingram David , Smith Arthur R.
Published: April 2011

Combination of tetrahedral and octahedral based nitrides are explored. The two cases of MnGaN and ScGaN with low Mn and Sc fractions are examined. It is found that for the MnGaN case, the Mn is incorporated under N rich conditions with little lattice change. However, for the ScGaN case, the Sc is incorporated onto the Ga sites but with a local bond angle distortion.
Nanostructured Energetic Materials with Sol-gel Methods
Authors: Gash Alexander E. , Satcher Joe H. , Simpson Randall L. , Clapsaddle Brady J.
Published: April 2011

The utilization of sol-gel chemical methodology to prepare nanostructured energetic materials as well as the concepts of nanoenergetics is described. The preparation and characterization of two totally different compositions is detailed. In one example, nanostructured aerogel and xerogel composites of sol-gel iron (III) oxide and ultra fine grained aluminum (UFG Al) are prepared, characterized, and compared to a conventional micron-sized Fe2O3/Al thermite. The exquisite degree of mixing and intimate nanostructuring of this material is illustrated using transmission and scanning electron microscopies (TEM and SEM). The nanocomposite material has markedly different energy release (burn rate) and thermal properties compared to the conventional composite, results of which will be discussed. Small-scale safety characterization was performed on the nanostructured thermite. The second nanostructured energetic material consists of a nanostructured hydrocarbon resin fuel network with fine ammonium perchlorate (NH4ClO4) oxidizer present.
The Catalytic Effect on Vertically Aligned Carbon Nanotubes
Authors: Kim Nam Seo , Bae Seung Yong , Park Jeunghee
Published: April 2011

We report the catalytic effect on the synthesis of multiwalled carbon nanotubes (CNTs). The CNTs were grown vertically aligned on the iron (Fe), cobalt (Co), and nickel (Ni) catalytic nanoparticles deposited on alumina substrates by thermal chemical vapor deposition (CVD) of acetylene in the temperature range 900–1000 °C. We also synthesized them on the silicon oxide substrates by pyrolyzing iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc), and nickel phthalocyanine (NiPc) at 700–1000 °C. In both syntheses, the CNTs grown using Fe exhibit about 2 times higher growth rate than those using Co and Ni. As the temperature rises from 700 to 1000 °C, the growth rate of CNTs increases by a factor of 45. The Arrhenius plot of growth rates provides the activation energy 30 ± 3 kcal/mol for all three catalysts, which is similar with the diffusion energy of carbon in bulk metal. It suggests that the bulk diffusion of carbon would play a decisive role in the growth of CNTs. The diameter of CNTs is in the range of 20–100 nm, showing an increase with the temperature. As the diameter is below 30 nm, the CNTs usually exhibit a cylindrical structure. The CNTs were intrinsically doped with the nitrogen content 2–6 atomic%. The degree of crystalline perfection of the graphitic sheets increases with the temperature, but depends on the catalyst and the nitrogen content. The graphitic sheets of CNTs grown using Fe are better crystalline than those grown using Co and Ni. As the nitrogen content increases, the degree of crystalline perfection decreases and the structure becomes the bamboolike structure probably due to a release of strains.
Ignition and Combustion Behaviors of Nanocomposite Al/MoO3
Authors: Granier John J. , Pantoya Michelle L.
Published: April 2011

Flame propagation in Al/MoO3 thermites is measured as a function of bulk density and initial sample temperature. The composites are composed of nano-scale reactant particles mixed and pressure molded to between 49 and 73% of the theoretical maximum density. A relationship between cylindrical die pressure and final pellet density is derived. Experiments are also performed by initially pre-heating samples to a uniform temperature ranging from 20 to 200 °C. Ignition sensitivity was determined by measuring the ignition delay time and temperature using a 50-W CO2 laser and thermocouples, respectively. Combustion wave speeds were measured using high-speed imaging diagnostics. Results allow comparison of combustion behaviors associated with nano- vs. micron-scale particle composites. The nano-scale particle composites are extremely sensitive to ignition, especially when initially preheated. Combustion wave speeds for the compressed nano-composites were found to double when compared to the micron-scale composites.
Underwater Explosive Behaviour of Compositions Containing Nanometric Aluminium Powder
Authors: Bates Leslie R
Published: April 2011

Results from research aimed towards the understanding of the effect that ultrafine aluminium powder has on the explosive performance of underwater explosives are presented. A series of aluminised explosive compositions using a commercially available nanometric aluminium powder (Alex) have been prepared and quantified underwater. The results have been compared with the corresponding compositions using conventional aluminium powders. An enhancement in explosive properties can be seen. The effect is not universal but can be related to the reaction zone length, and time, of the base explosive being sufficiently long for the oxidation of the aluminium, with its substantial energy release, to take place in or close to the reaction zone.
Design of High-Energetic Materials at the Nanoscale
Authors: Rao Bijan K. , Jena Purusottam
Published: April 2011

The amount of energy storage and its release in controllable pathways are two of the fundamental requirements of a high-energy material. The novel chemistry brought about by large surface-to-volume ratio of nanomaterials provides an attractive way to design and synthesize materials that optimize these two requirements. First principles calculations based on density functional theory and generalized gradient approximation have been used to study the potential of AlxLiyOz and Al(MnO4)x clusters as candidates for high-energetic materials. The equilibrium geometries and total energies of these clusters and their fragments are obtained to study the energy stored in these clusters and its release along various pathways. Interesting results include the substantial increase in binding energy by either adding an Al to MnO4 or adding a MnO4 to Al(MnO4)x unit indicating that Al(MnO4)3 may be a potential candidate for energetic materials as well as super-oxidizers. Similar calculations also show that during the combustion of Al addition of small amounts of Li to Al nano-powder helps to reduce the amount of non-combustible Al.
Molecular Dynamics Studies of Nanoparticles of Energetic Materials
Authors: Alavi Saman , Velardez Gustavo F. , Thompson Donald L.
Published: April 2011

The structural properties of several nanoparticles of 2,4,6,8,10,12-hexanitrohexaazaiso-wurtzitane, HNIW or CL-20, are studied by using molecular dynamics simulations. The internal structure of the CL-20 molecule is held rigid and the intermolecular interactions in the nanoparticles are taken from a previously developed force field. [Sorescu et al., J. Phys. Chem. B, 102, 948 (1998)] Molecular dynamics simulations of solid-like and annealed nanoparticles with 48 and 88 CL-20 molecules have been carried out in the solid-state range of temperatures from 50 to 500 K. The center-of-mass to center-of-mass radial distribution functions, dipole-dipole correlation function, the orientations of the surface dipoles, and the density of the nanoparticles were calculated at fixed temperatures for the nanoparticles.
High-Resolution TEM Characterization of Carbon Aerogels as Catalyst Supports
Authors: Kang Dafei , Zhang Ying , Saquing Carl , Erkey Can , Aindow Mark
Published: April 2011

Carbon aerogels with tailored pore sizes were prepared by using a combination of sol-gel processing, supercritical CO2 extraction and pyrolysis. Ruthenium particles were incorporated via impregnation of organometallic precursors using supercritical CO2, followed by in situ thermal reduction. TEM studies reveal that the carbon aerogels are characterized by a continuous pore structure and that the ruthenium particles are highly dispersed with a narrow size distribution. It is found that under certain circumstances the carbon aerogels undergo a structural transition under the electron beam, in which the original tortuous aerogel network transforms into onion-like structures.
Nanofibrous Manganese Dioxide for Decomposition of Volatile Organic Compounds
Authors: Chen H. , Xiao T. D. , Liu S.
Published: April 2011

Volatile organic compounds (“VOCs”) are widely used in industrial process, transportation fuels, and households. It is inevitable that some VOCs will escape into the atmosphere. Emission of VOCs will cause three major problems, including (1) increased exposure level of the population to know carcinogenic compounds such as benzene and carbon tetrachloride, (2) increased ozone concentration in the atmosphere at ground level, and (3) stratospheric depletion of the ozone layer, which alters global climate and increases exposure to UV radiation. Reducing the concentrations of VOCs will greatly improve air quality. A common method for VOC removal is catalytic decomposition. Most works have only focus on reducing VOC concentrations down to a few parts per million (ppm). Very little research has been conducted on reducing the VOCs down to parts per billion (ppb) levels, or less. This paper reports the decomposition of VOCs using a unique class of nanofibrous bird's-nest superstructure material. This open-inter-woven nanofibrous superstructure provides superb host for catalytic reactions and permits relatively easy flow of the gas stream through the structure. The nanofibrous MnO2 is highly effective in destroy more than 50 VOCs on USEPA's top priority list toxic VOCs to ppb level or less at a 200 − 350°C temperature.
Hydriding Behaviour of Mg-C Nanocomposites
Authors: Bassetti A. , Bonetti E. , Fiorini A. L. , Grbovic J. , Montone A. , Pasquini L. , Antisari M. Vittori
Published: April 2011

Magnesium carbon nanocomposites for hydrogen storage have been synthesized by ball milling with different amount of benzene, acting as a lubricant. Their microstructure has been studied by X-ray diffraction and scanning electron microscopy, while the hydrogen desorption temperature has been tested by differential scanning calorimetry. Experimental results show that the microstructure after milling, the hydrogenation capabilities of the material and the reactivity with the air are related to the amount of additives. In particular the carbon to benzene ratio seems to play a major role. In fact, with an optimum value of carbon to benzene weight ratio of 1/6, the amount of carbon being 15 wt% of the milled mixture, a decomposition heat equal to 57% of pure MgH2 was measured, even after air manipulation of the sample.
Hydrogenation of nanocrystalline Mg-based alloys
Authors: Zander Daniela , Lyubenova Lyudmilla , Köster Uwe , Klassen Thomas , Dornheim Martin
Published: April 2011

Desirable properties of hydrogen storage in materials include mainly, high hydrogen capacity, high number of hydride / dehydride cycles, low temperature of dissociation and microstructural stability. Hydrogen storage can proceed from the gas phase as well as during cathodic charging.

The use of nanocrystalline Mg powder without and with 2 mol% Nb2O5 catalyst as an electrode material for electrochemical hydrogen charging processes in a 6M KOH electrolyte was studied. A strong influence of the compaction parameters and the current density but no influence of the catalyst on the hydrogenation behavior was observed. However, a strong influence of the catalyst on the hydrogen overpotential was observed and is assumed to result into a change of the electrochemical surface reaction. The addition of graphite and PTFE to the Mg/Nb2O5electrodes improves the charging kinetic as well as the hydrogen content up to 1 wt.% hydrogen.
ab-initio modelling of Mg:H interstitial solid solutions
Authors: Messina M. , Cleri F. , Volpe M.
Published: April 2011

We studied the thermodynamics of interstitial Mg:H solid solutions by means of ab-initio electronic structure calculations. Soft pseudopotentials (Troullier-Martins) with non linear core correction and Perdew-Burke-Ernzerhof GGA exchange-correlation functional were employed, in the framework of a DFT plane-wave scheme. We inserted increasing concentrations of interstitial H atoms in the hcp Mg lattice, in both tetrahedral and octahedral positions. We calculated the heat of solution and the volume variation as a function of H concentration. Although the difference in Gibbs free energy is positive at any H concentration above 10−6 at.%, our results show that the enthalpic contribution is negative at any H concentration, therefore locally favouring H clustering. This reflects the existence of a driving force for the subsequent formation of the hydride. The volume deformation was characterized, finding that octahedreal interstitials have a minor effect, while tetrahedral interstitial can induce very large local expansion.
Hydrogen Storage using Carbon Materials at Room Temperature
Authors: Kajiura H. , Tsutsui S. , Kadono K. , Murakami Y. , Kakuta M. , Ata M.
Published: April 2011

The hydrogen adsorption capacity of commercially available carbon materials with different nanostructures was measured at room temperature using an apparatus based on a volumetric method with an error of less than 0.04 wt% per one gram of sample. The obtained results suggest that nanosutures of the sample influence the hydrogen adsorption capacity. To confirm this hypothesis, we prepared nanostructured graphite from graphite powder using a mechanical milling process at a pressure of 2.0 × 10−4 Pa. The untreated graphite adsorbed 0.02wt% of hydrogen at 6 MPa at room temperature, while 0.20 − 0.25 wt% of hydrogen can be repeatedly adsorbed by the nanostructured graphite. Measurements of the hydrogen adsorption rate at constant pressure and pore-size distribution imply that the hydrogen molecules are adsorbed through a diffusion process in pores with a diameter less than 1 nm.
The Electrical Characteristics of a Pd/AlN/Si Thin Film Hydrogen Gas Sensor
Authors: McCullen Erik F. , Prakasam Haripriya E. , Mo Wenjun , Thakur Jagdish , Naik Ratna , Rimai Lajos , Ng K.Y.S. , Auner Gregory W.
Published: April 2011

We have extended our previous investigation of the electrical characteristics of a Pd/AlN/Si thin film sensor for varying thicknesses of AlN, from 300–2000Å. The capacitance vs. voltage, C(V), and conductance vs. voltage, G(V), measurements were utilized to investigate the presence of surface states within the Si gap at the AlN/Si interface. Our previous experiments on 500Å AlN did show the presence of interface traps, with an estimated surface density between 8×1014 and 1.5×1015 m−2eV−1 [1]. In our present work we've examined the effect of AlN thickness on the density of these interface traps. The density is dependent on AlN thickness. The thinner devices, 300Å, showed an interface trap density of 20–30×1015 m−2eV−1. The interface trap density decreased with increasing thickness up to 500Å, where the density remained relatively constant at about 1–5×1015 m−2eV−1 for thicknesses up to 2000Å. We have also shown that the interface trap density is independent of annealing.
Studies on the perovskite-based La4Srn−4TinO3n+2
Authors: Canales-Vázquez Jesús , Tao Shawen , Patrakeev Mikhail , Irvine John T. S. , Zhou Wuzong , Frade Jorge
Published: April 2011

Perovskite titanates with nominal stoichiometry ABO3+Δ are frequently reported in SOFC literature as having O3 stoichiometry. Such phases often exhibit quite interesting properties, but are not stoichiometric and the structural characterisation is usually not rigorous. Here we demonstrate how oxygen excess can be incorporated in a titanate perovskite-based lattice as crystallographic oxygen shears giving rise to the homologous series La4Srn−4TinO3n+2. The layered structure is lost for the n=12 member (La4Sr8Ti12O38−Δ), although the oxygen excess is accommodated within the perovskite framework in randomly distributed short-range linear defects. This compound was studied as potential fuel electrode due to its high total conductivity and stability in the most reducing conditions.
The importance of high temperature electron-phonon coupling to the thermodynamic properties of Ce0.9Th0.1 and other f-electron bonded metals
Authors: Manley M. E. , McQueeney R. J. , Fultz B. , Swan-Wood T. , Delaire O. , Goremychkin E. A. , Cooley J. C. , Hults W. L. , Lashley J. C. , Osborn R. , Smith J. L.
Published: April 2011

Recent results from inelastic neutron scattering studies on Ce0.9Th0.1 are used to demonstrate the importance of electron-phonon coupling to the physical properties of f-electron bonded metals. In the case of Ce0.9Th0.1, the phonon density of states (DOS) of α-phase shows a significant softening when heated but shows no change across the α-γ transition despite the 11% volume change. This is supported by analysis of the magnetic spectra showing that most of the transition entropy can be accounted for with the crystal field and spin fluctuations. The precursor phonon softening, the lack of any phonon change across the transition, the magnetic spectra, and the volume transition itself can all be explained in terms of the atomic displacement sensitivity of the hybridization of the local f-electrons with conduction electrons. The electron-phonon coupling resulting from these displacement-sensitive electronic states appears to be essential to understanding cerium. Some of the behavior characteristic of these states, a large volume changes and precursor phonon softening, occurs in many other f-electron bonded metals suggesting that the phenomena is not unique to cerium.
Study of Actinides Incorporation in Thorium Phosphate-Diphosphate/Monazite Based Ceramics
Authors: Clavier Nicolas , Dacheux Nicolas , Podor Renaud , Le Coustumer Philippe
Published: April 2011

Phosphate materials are usually considered as potential candidates to perform the immobilization of actinides coming from an advanced reprocessing of spent fuel in the field of an underground repository. Among them, Thorium Phosphate-Diphosphate (TPD) and monazites have been already extensively studied. The elaboration of TPD/monazite based materials was thus envisaged in order to immobilize simultaneously tri- and tetravalent actinides and a neutron absorber. Two chemical ways of synthesis were considered and the compounds were easily prepared in the powder and in the pellet form. A good chemical compatibility was found between TPD and monazite since the properties of both phosphates were kept in these composites. Moreover, the relative density of the pellets reached 90 – 95 % of the value calculated from XRD data. The normalized dissolution rates determined in acidic media did not exceed 5.10−4 g.m−2.day−1 which confirmed the very good durability of such materials during leaching tests.
On the Physical Nature of Uranyl Charge Transfer Vibronic Interactions
Authors: Chen X. Y. , Rao L. F. , Liu G. K.
Published: April 2011

We address the electronic properties of uranyl ions in solids and solutions with an emphasis in theoretical understanding of charge transfer vibronic transitions and luminescence dynamics the O-U-O species. A general theory of ion-phonon interaction has been modified for modeling and simulating multi-phonon vibronic spectra. Spectroscopic data for uranyl ions in crystals and solutions have been analyzed to achieve a predictive understanding of the uranyl-ligand vibronic interactions. By adjusting the Huang-Rhys ion-phonon interaction parameters, an excellent agreement between theory and experiment has been accomplished for uranyl ions in the ligand environments we studied. Our modeling and simulation provide insights into the physical nature of uranyl vibronic interaction and its influence on spectroscopic properties, which are commonly utilized in characterizing photochemical properties of uranyl in complexes.
Cohesive Properties of PuGa Alloys
Authors: Robert Gregory , Colinet Catherine , Siberchicot Bruno , Pasturel Alain
Published: April 2011

We present results on the structural stability of PuGa alloys based on the local spin-density approximation and including non-local corrections to the exchange-correlation functional. We use the projector augmented wave (PAW) method to calculate the formation energies of five intermediate phases. The stability of the δ solid solution is analysed using selected supercells for total energy caclulations. Cluster variation calculations in the tetrahedron approximation are performed to study the effect of composition and of finite temperature on the ordering phenomena in the δ solid solution. The Pu-rich part of the Pu-Ga phase diagram is computed.
SIC-LSD study of δ-Pu and PuO2±x
Authors: Petit L. , Svane A. , Szotek Z. , Temmerman W. M.
Published: April 2011

The electronic structures of actinide solid systems are calculated using the self-interaction corrected local spin density approximation. Within this scheme the 5f electron manifold is considered to consist of both localized and delo-calized states, and by varying their relative proportions the energetically most favourable (ground state) configuration can be established. Specifically, we discuss elemental Pu in its δ-phase, and the effects of adding O to PuO2.
Uranium (and Cerium) Compounds At High Pressures and Magnetic Fields
Authors: Cornelius Andrew L. , Kumar Ravhi S. , Light Brian E.
Published: April 2011

Correlated-electron systems are so named due to strong interactions between electrons unlike traditional metals (e.g. copper) that have “free electrons” that interact very weakly. Knowledge of the Fermi surface, density of electron states and band structure are the starting points for a first-principles understanding of the electronic and electronically related macroscopic properties, e.g. equation of state. The use of high pressure and high magnetic fields to alter the electron-electron (hybridization) and electron-lattice interactions give us powerful tools to understand complicated rare earth and actinide correlated-electron systems and allows precise testing of experiment to theory. Correlated-electron systems yield a wide variety of ground states that are a direct result of the hybridization strength including: short and long range magnetic order, spin fluctuating, enhanced Pauli paramagnetism, heavy fermion behavior and superconductivity. We will review some results on U compounds in high magnetic fields and high pressures. By comparing the results to Ce compounds that have significantly more localized f electrons, the effect of direct 5f electron wavefunction overlap in U compounds can be discerned. Consequences on the search for U based heavy fermion superconductors will be discussed.
High Throughput Screening of Materials for Solid Oxide Fuel Cells
Authors: Lemmon John P. , Manivannan Venkatesan , Jordan Tracey , Hassib Lamyaa , Siclovan Oltea , Othon Michelle , Pilliod Mile
Published: April 2011

State of the art commercial cathodes for solid oxide fuel cells (SOFC) include LaMnO3 with a zirconia-based electrolyte. However, the vacancy concentration in A site doped LaMnO3 is low, thus ionic conductivity is also very low (10−7 – 10−8 S/cm at 800 °C). The surface path dominates the reaction rate of the LaMnO3 cathode; therefore the optimized electrode is a porous composite material of both the cathode material and electrolyte and relies on triple-point boundaries for performance. The electrical conductivity and thermal expansion properties of this cathode material and other A3+B3+O3 perovskites can be tuned by substitution at the A and/or B site. The numerous combinations of composition, processing and microstructure needed for improved cathode performance is well suited for a high throughput screening (HTS) approach towards optimization and discovery. We present here a high throughput discovery process that includes, synthesis, performance testing and characterization techniques directed towards new low temperature SOFC cathode materials.
Combinatorial Chemical Vapor Deposition of Metal Silicate Films Using Tri(t -butoxy) silanol and Anhydrous Metal Nitrates
Authors: Zhong Lijuan , Chen Fang , Campbell Stephen A. , Gladfelter Wayne L.
Published: April 2011

A modified low-pressure chemical vapor deposition reactor was used to create compositional spreads of MO2/SiO2 films (M = Hf, Zr and Sn) using tri(t-butoxy) silanol and anhydrous metal nitrates of hafnium, zirconium and tin at temperatures below 250 °C. The compositional spreads formed by this process were characterized by ellipsometry and Rutherford backscattering spectrometry. A survey of possible reactions involved in the deposition is included.
Combinatorial Fabrication and Studies of Small Molecular Organic Light-Emitting
Authors: Li Devices G. , Zou L. , Cheon K. O. , Shinar J.
Published: April 2011

Various combinatorial matrix arrays of UV-violet, white, and blue-to-red organic light-emitting devices (OLEDs), fabricated using a sliding shutter technique, are described. In the UV-violet devices, which contain a UV-violet emitting layer of 4,4′-bis(9-carbazolyl) biphenyl (CBP), the optimal radiance R and external quantum efficiency ηext were determined with respect to the thicknesses of the hole transporting layers. In the blue-to-red devices, which contained a blue-emitting layer of 4,4′-bis(2,2′-diphenyl-vinyl) -1,1′-biphenyl (DPVBi) and a red-emitting 5 wt.% dye-doped guest-host layer, the color of the devices evolved continuously from blue to red as the thickness of the doped layer increased from 0 to 35 Å. The (nominal) 2 Å-thick doped layer device exhibited the highest brightness L ∼ 120 Cd/m2 and external quantum efficiency ηext ∼4.4 % at a current density of 1 mA/cm2. In the white OLEDs, which were similar to the blue-to-red devices but with lightly doped emission layer, the highest brightness Lmax was over 74,000 Cd/m2; in all devices Lmax exceeded 50,000 Cd/m2. The maximum efficiencies were 11.0 Cd/A, 5.96 lm/W and 4.6% at 5.8 V, 0.6 mA/cm2, and 68 Cd/m2 in a 0.25 wt.%, 2 nm-thick doped layer device.