bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantum Confinement Effects on 100–400 Å Diameter Silver Bromide Microcrystals
Authors: Ilana Freedhoff Michal , McLendon George , Marchetti Alfred
Published: April 2011

Previous work on quantum confined AgBr microcrystals has been limited to either large (500-1000 Å diameter) aqueous samples or extremely small (60-100 Å diameter) inhomogeneously sized micellar samples. This has prevented an investigation of the effect of size restriction on the phonon assisted transitions of the indirect exciton. A non-aqueous preparation was developed, capable of providing more homogeneous size distributions of AgBr microcrystals in the 100-400 Å diameter range. Low temperature luminescence studies were performed to determine the changes in energy and intensity associated with these excitonic transitions. The radiative lifetimes of some of the excitonic processes were measured in order to obtain a more global perspective on the photophysics of AgBr.
Electronic Properties of Porous Silicon
Authors: Beltran M. R. , Tagüeña-Martinez J. , Cruz M. , Wang C.
Published: April 2011

Porous silicon (PS) has been extensively studied in recent years. The origin of its luminescence has been a subject of debate. This work attempts to give some insight towards the understanding of this phenomenon, studying the behaviour of the energy band structure of PS as a function of the pore morphology and distribution, for a given porosity. The porous structure is modeled as empty columns of different sizes and shapes, produced into san otherwise silicon perfect crystal. The columns are passivated with hydrogen atoms. A tight-binding Hamiltonian on an ; sp3:s* basis set is applied onto a supercell. Due to the simplicity of the model, morphology effects can be analyzed. The results show that the band :gap and the nature of the 'States at the top of the valence band depend on the morphology. Furthermore, we also discuss the shift of the conduction band minimum towards the gamma point, producing an almost direct band gap, as the pore distribution changes.
Extended Quantum Model for Porous Silicon Formation
Authors: Münder H. , Frohnhoff St. , Berger M.G. , Marso M. , Thönissen M. , Arens-Fischer R. , Lüth H.
Published: April 2011

The formation of porous silicon (PS) by electrochemical dissolution of bulk Si is described by a new model involving quantum mechanical calculations of the tunneling probability of holes through small crystallites (< 60 Å) into the electrolyte. This tunneling probability shows oscillations as a function of crystallite size. The presented model calculations are in agreement to the microstructure of p-PS — deduced from Raman measurements — as a function of etching parameters and substrate doping level.
Non-Destructive Characterization of Porous Silicon Using X-Ray Reflectivity
Authors: Chason E. , Guilinger T.R. , Kelly M.J. , Headley T.J. , Howard A.J.
Published: April 2011

Understanding the evolution of porous silicon (PS) layers at the early stages of growth is important for determining the mechanism of PS film growth and controlling the film properties. We have used X-ray reflectivity (XRR) to determine the evolution of layer thickness and interfacial roughness during the growth of thin PS layers (< 200 nm) prepared by electrochemical anodization. The porous layer grows at a constant rate for films as thin as 15 nm indicating a very short incubation period during which the surface may be electropolished before the PS structure begins to form. Interface roughness measurements indicate that the top surface of the film remains relatively smooth during growth while the roughness of the PS/silicon interface increases only slightly with film thickness. The XRR results are compared with results obtained from the same films by cross-sectional transmission electron microscopy (XTEM), atomic force microscopy (AFM) and gravimetry.
Formation and Properties of Porous Si Superlattices
Authors: Berger M.G. , Arens-Fischer R. , Frohnhoff St. , Dieker C. , Winz K. , Münder H. , Lüth H. , Arntzen M. , Theiss W.
Published: April 2011

Porous silicon superlattices (PS-SL) were investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), spectroscopic ellipsometry and reflectance spectroscopy. Type I superlattices were formed by periodically changing the current density during the etch process. The quality of the interface between layers of different porosity depends on the quality of the silicon substrate. Optical components such as Bragg reflectors or Fabry-Perot filters were designed using the optical data of single porous layers. A good long term stability of the layers is achieved by using thermal oxidation steps. Type II superlattices were formed on substrates with layers of alternating doping level. The more complex layer structure of these superlattices is explained by the selectivity of the etch process on the doping level.
Preparation, Properties and Applications of Free-Standing Porous Silicon Films
Authors: Von Behren J. , Tsybeskov L. , Fauchet P. M.
Published: April 2011

Using special electrochemical etching and lift-off steps, we have fabricated large-area freestanding porous silicon films in the thickness range from 0.1 μm to 50 μm. Their transmission is near 100% in the near infrared which is indicative of very high porosity/low index of refraction films. These optically flat and homogeneous films exhibit no surface and bulk scattering, despite the fact that they did not undergo supercritical drying. The relationship between the absorption coefficient, the luminescence spectrum, and the chemical and structural properties is examined as a function of preparation and post-treatment conditions. Because of their superior optical properties, these films are suitable for many device applications.
Effective Dielectric Function of Porous Silicon: the Transverse Component
Authors: Gumbs G.
Published: April 2011

A self-consistent many-body theory is developed to study the effect of temperature and electron density on the interband absorption coefficient and the frequency-dependent refractive index for an array of isolated quantum wires. The peaks in the absorption coefficient correspond to interband transitions resulting in the resonant absorption of light. The oscillations in the derivative spectrum are due to the quantization of the energy levels related to the in-plane confining potential for such reduced dimensional systems. There are appreciable changes in the absorption spectrum when the electron density or temperature is increased. One interband transition peak is suppressed in the high electron density limit and the thermal depopulation effect on the electron subbands can be easily seen when the temperature is high. We also find that the exciton coupling weakens the shoulder features in the absorption spectrum. This study is relevant to optical characterization of the confining potential and the areal density of electrons using photoreflectance. By using incident light with tunable frequencies in the interband excitation regime, contactless photoreflectance measurements may be carried out and the data compared with our calculations. By fitting the numerical results to the peak positions of the photoreflectance spectrum, the number of electrons in each wire may be extracted.
Localized States and Porous Silicon Luminescence
Authors: Dubin V. M. , Ozanam F. , Chazalviel J.-N.
Published: April 2011

Electronic states of photocarriers in porous silicon have been investigated by photomodulated infrared and photoluminescence spectroscopies. Wet porous silicon is only weakly yellow-green luminescent, and contains photocarriers which exhibit a Drude-like absorption characteristic of free carriers. On the contrary, once dried or anodically oxidized, porous silicon becomes strongly red luminescent and photocarriers exhibit a broad Gaussian-like absorption, characteristic of localized carriers, which correlates in frequency, response time and intensity with the photoluminescence. This behavior does not appear to be related to surface chemistry. Instead, non-wetting of the surface seems to be mandatory in order to obtain localization of the photocarriers and strong red luminescence. This suggests that the driving effect for switching from green to red luminescence could be the dielectric screening of the Coulombic interactions by the embedding medium in which porous silicon is maintained. The electronic states bound to the potential generated by a charge in a cylindrical silicon wire have then been computed for various values of the dielectric constant of the embedding medium. It is shown that, when the dielectric constant is low, one may account for the red luminescence in terms of recombination through such shallow states, and that the finite time needed for the dielectric relaxation in electrolytes may also account for an inefficient trapping in such states in a wet environment. Preliminary luminescence results are consistent with the predictions of the model.
Experiments and Monte Carlo Simulations on the Recombination Dynamics in Porous Silicon
Authors: Pavesi L. , Roman H. Eduardo
Published: April 2011

We present a detailed study of the time-resolved photo-luminescence of porous Silicon samples with different porosities providing clear evidence of anomalous relaxation behaviour of the luminescence, which follows stretched exponential decay for a variety of experimental conditions. In addition, a numerical study of the underlying transport behaviour in these disordered materials by means of Monte-Carlo simulations has been performed. Nanometer sized particles, characterised by a distribution of radiative and non-radiative recombination times, are randomly placed at the sites of a cubic lattice forming a single three dimensional percolation cluster. Charge carriers are allowed to hop between nearest-neighbour occupied sites. The competing effect between radiative and non-radiative transitions in a single nanometer particle, as well as the effects of geometrical constraints on transport due to the complex topology, are discussed and compared to experiments.
Temperature Dependence of Stresses and H Desorption in Porous Silicon
Authors: Diawara Y. , Currie J.F. , Yelon A. , Petrova-Koch V. , Nikolov A.
Published: April 2011

The mechanical stress induced in as-prepared porous silicon is generally compressive. Upon thermal annealing, the stress shows hysteresis. Isothermal annealing reveals second-order kinetic processes for both the stress and hydrogen effusion. The correlations between the changes in stress and porous silicon composition and structure are discussed.
Gas Sensor Using an Aluminium-Porous Silicon Junction Application to the Detection of Non-Zero Molecular Dipole Moment
Authors: Stievenard D. , Deresmes D.
Published: April 2011

Porous silicon is known to be sensitive to moisture. Using an aluminium-porous p+ silicon junction, we have realized a sensor which dc current increases up to two orders of magnitude in the presence of ammoniac. We have tested a series of various gases and we show that if the dipole moment of the molecule is zero, there is no effect on the dc current. To interpret quantitatively this phenomenon, we assume that the conductivity is governed by the width of a channel resulting from the partial depletion of silicon located between two pores. This depleted region is due to the charges trapped on surface states associated with the Si-SiO2 interface where SiO2 is the native silicon oxide. When some gas is adsorbed, we propose there is a passivation of the interface states (mainly dangling bonds), leading to a decrease of the depleted region, i.e. an increase of the width of the channel and thus an increase of the current. The adsorbed gas gives a dipole layer at the surface of the pore. This layer has no influence on the depleted region. It stabilizes electrons or holes at the porous Si surface, allowing a stable charge state of the dangling bonds.
Photoluminescence and Electroluminescence in Partially Oxidized Porous Silicon
Authors: Tsybeskov L. , Duttagupta S. P. , Fauchet P. M.
Published: April 2011

The results of photoluminescence (PL) and electroluminescence (EL) studies from partially oxidized porous silicon (POPS) layers are presented. The PL from POPS is stable, peaks at 600-570 nm and its temperature dependence can be fitted by an exponential law with an activation energy Ea « 10 meV. The current-voltage characteristics of Au-(POPS)-crystalline silicon (c-Si) structures follow a power law I = Vn. When the index n becomes higher than 3, electroluminescence (EL) is found. The EL peaks at 760 nm and is stable for more than 100 hours of operation. The intensity of the EL is a linear function of current for all measured structures up to current density J ≈ 1 A/cm2. Our results suggest that partially oxidized porous silicon is more useful for device applications than freshly anodized porous silicon which has unstable properties or than fully oxidized porous silicon in which transport is poor.
Preparation of Nanocrystalline Silicon by Pulsed Plasma Processing
Authors: Oda S. , Otobe M.
Published: April 2011

We have proposed digital plasma processing for the fabrication of silicon quantum dots with grain size less than l0nm. By using the pulsed gas supply of SiH4 and H2 in the very-high frequency (VHF) plasma, we have clarified the role of atomic hydrogen in the nucleation, crystallization of nanocrystalline Si (nc-Si) as well as in the selective etching of amorphous Si to nc-Si. Recently, we have prepared nc-Si by employing an ultra-high-vacuum (UHV) chamber equipped with VHF plasma cells of SiH4 and H2. Flux rate of Si cluster depends significantly on the pressure of the plasma cell and VHF power. Spherical shaped nc-Si clusters less than 6nm in diameter have been observed by transmission electron microscopy (TEM). Infrared absorption measurements have clarified that the surface of nc-Si is covered by hydrogen. In an attempt to control the position of nuclei, we have prepared nc-Si on SiO2 with micro trenches, 40nm wide and 20nm deep, fabricated by electron beam exposure and electron cyclotron resonance (ECR) etching. It has been revealed by TEM observation that nc-Si are formed preferentially along micro trenches.
Thin Films of Semiconducting SnSi Alloys Grown by Pulsed Laser Deposition
Authors: Treece Randolph E. , Horwitz J. S. , Chrisey D. B. , Tang J. , Williams R. S.
Published: April 2011

Semiconducting SnxSi1−x (0≤x≤0.6) thin-film alloys have been grown by pulsed laser deposition (PLD). These new materials are amorphous to X-rays and display small positive optical band gaps, suggesting potential applications in solar cells. The tin silicide films were grown by depositing very thin (1–30 Å) alternating atomic layers from individual Sn and Si targets utilizing an automated multi-target holder coupled to a conventional PLD system. The value of x was selected by controlling the thickness of the atomic layers. The films were characterized by X-ray diffraction, optical absorption, Rutherford backscattering spectroscopy, temperature-dependent resistivity, and X-ray photoelectron spectroscopy. Tin segregation is prevented by keeping the Sn layer thickness below a critical value. Compositions beyond x > 0.6 led to semimetallic SnxSi1−x films with tin crystallites.
Transmission Electron Microscopy Observation of Constrained Crystallization in a-Si:H/a-SiNx:H Multilayer Film
Authors: Huang Xinfan , Shi Weihua , Chen Kunji , Yu Shidong , Feng Duan
Published: April 2011

The constrained crystallization in a-Si:H/a-SiNx:H multilayer structures by Ar ion laser annealing treatment has been studied by high-resolution transmission electron microscopy (HRTEM) and Raman scattering. HRTEM photograph shows that the a-Si:H layers crystallize without disturbing the multilayer structures and that the interfaces after the crystallization are atomically smooth and uniform. The lattice image of the Si crystallites arrayed one by one can be seen clearly in Si layers and the average size is roughly equal to the thickness of Si layer. The thermodynamics of constrained crystallization within multilayer structures has been discussed.
Nd-YAG Laser Induced Crystallization on a-Si:H Thin Films
Authors: Carvalho J. , Ferreira I. , Fernandes B. , Fidalgo J. , Martins R.
Published: April 2011

In this paper we present results concerning the influence of laser energy and shot density on the electrical resistance, X-ray diffraction pattern, and structure obtained by SEM, on recrystallized a-Si:H thin films produced by using a Nd-YAG laser, working in a wavelength of 532 nm. The base material (undoped and doped a-Si:H) was obtained by Plasma Enhanced Chemical Vapour Deposition (PECVD). The structure and electrical characteristics of the recrystallized thin films are dependent on the laser beam energy density, beam spot size and the number of shots applied to the base a-Si:H thin film used. Overall, the data show recrystallized material with grain sizes larger than lµm, where the electrical resistance of both, undoped and doped materials, can be varied up to 5 orders of magnitude, by the proper choice of the recrystallization conditions.
Structural and Electrical Characterization of Undoped Poly-Si Oxides
Authors: Sakamoto T. , Tokioka H. , Takanabe S. , Kubota T. , Niwano Y. , Goto Y. , Namizaki H. , Wada O. , Kurokawa H.
Published: April 2011

We evaluated structural and electrical characteristics of undoped poly-Si oxide films. Poly-Si films made by solid phase crystallization at 600-900°C from undoped amorphous Si films were oxidized to form oxide layers of 140nm thickness. We observed protuberances on the surface of poly-Si layers after oxidation. Poly-Si oxide layers also generated protuberances above the protuberances of poly-Si films. The number of protuberances per unit area is larger in the case of high temperature crystallization. The measurement of current through the poly-Si oxide films shows that the conductivity of poly-Si oxide films depends on crystallization temperature of poly-Si films in the case of positive gate bias. When the gate is biased negatively, current through the poly-Si oxide films remained almost constant regardless of crystallization temperature. We find that poly-Si crystallized at lower temperatures offers poly-Si oxide films of lower leakage current in the case of electron injection from undoped poly-Si layers. The lower leakage current is due to highness of energy barrier for electron at undoped poly-Si/poly-Si oxide interface.
Control of the Crystallite Size and Dielectric Tissue in nc-Si/SiO2 Plasma CVD Films: Origin of the Green/Blue and the Efficiency of the Red Photoluminescence
Authors: Vepřek S. , Wirschem Th. , Rückschloß M. , Tamura H. , Oswald J.
Published: April 2011

Plasma CVD allows one a good control of the crystallite size and dielectric tissue in the nc-Si/SiO2 compact films. It is shown that the green/blue PL originates from SiO2 (s) ≡Si-OH groups whereas the red PL is associated with states at the nc-Si/SiO2 interface. With ≥ 1 nm thick SiO2 tissue the efficiency of the red PL increases by more than three orders of magnitude when the crystallite size decreases from about 8 to 2 nm in agreement with the theoretical predictions. If, at a constant crystallite size, the thickness of the SiO2 tissue is decreased to 0.2-0.3 nm, the PL intensity decreases by two orders of magnitude. Possible reasons of this effect are discussed together with the question of optimizing the electroluminescence. Microwave absorption is used to distinguish between free and localized (emitting PL) photogenerated carriers.
Production of Fullerenes from Solar Energy
Authors: Laplaze D. , Bernier P. , Barbedette L. , Flamant G. , Lebrun M. , Brunelle A. , Della-Negra S.
Published: April 2011

The high intensity of solar radiation, obtained with the Odeillo (France) solar furnace facilities, is used to vaporize graphite in inert gas atmosphere. The soot obtained contains C60, C70 and other heavier fullerenes. We discuss the possibility of increasing the evaporation rate of graphite and the yield of soot with this technique. From our last experiments, we obtain a first estimate of the soluble fullerene yield Y ( greater than 12%) and we have shown that 13C enriched fullerenes can be easily produced by this process.
Smaller Carbon Clusters: Linear, Cyclic, Polyhedral
Authors: Slanina Z. , Lee S.-L. , Smigel M. , Kurtz J. , Adamowicz L.
Published: April 2011

The MP2 perturbation treatment with the 6-31G* basis set has been applied to linear, cyclic, and polyhedral structures of Cn, n = 6-13, esp. to C12 in this report. While the linear and cyclic species can co-exist, polyhedral ones are too high in energy. Preliminary results on C8H (related to diffuse interstellar bands) are reported, too.
Constraints on Small Fullerene Helices
Authors: Dunlap Brett. I.
Published: April 2011

A single heptagon together with a single pentagon can join two graphene semitubules together at a 30° angle so that all carbon atoms are three-fold coordinated and all other carbon rings are hexagons. This bend connects tubules of complementary classes. A tubule class is the set of all tubules having the same helicity. A tubule class has a uniform density of radii, which is the number of different tubules in the class per unit change in tubule radius. The classes that are joined by a heptagon and pentagon have smallest members whose radii differ by a factor of . Thus tubule segments joined by a heptagon and pentagon cannot have exactly equal circumferences. There are only a finite number of twist angles allowed between sequential bends along a tubule. Pentagons and heptagons should be isolated and separated as far as possible. These requirements particularly constrain the bends and twists that can occur in the smallest tubules. These considerations favor long-range order along a helix, given a driving force for curling during the formation process.
The Fullerene Neighbours
Authors: Slanina Z. , Sun M.-L. , Lee S.-L. , Adamowicz L.
Published: April 2011

Semiempirical quantum-chemical calculations are reported for the fullerenic structures C60, Si60, Ge60, N60; B36N24, B36P24, A136N24, A136P24; and various BnNn A new route towards B/N clusters is considered, being based on squares and hexagons. The pattern always requires six squares. The route can produce species of similar or even higher stability comparing to the conventional pentagon/hexagon pattern. Four particular stoichiometries emerge from the available AM1 computations: B12N12, B28N28, B36N36, and B36 N24.
Electronic Structure Calculations of Defect C60 with One or Two Vacancies
Authors: MorÁn-lÓpez J.L. , Dorantes-DÁvila J. , Cabrera-Trujillo J.M.
Published: April 2011

The electronic properties of defect C60 with one or two vacancies, are calculated by using a Hubbard-like Hamiltonian for sp-electrons in the unrestricted Hartree-Fock approximation. Results are given for the cohesive energy and local charge distribution of the different non-equivalent sites. These results might support a possible mechanism to encapsulate atoms in the internal cavities of C60. This mechanism involves the production of C60 molecules with two carbon isotopes AC and BC (A, B = 12,13,14). The molecules AC59BC1 and AC58BC2 are separated from the total production and collected in a chamber under partial pressure of the element to be inserted.
Thermodynamic Properties of the Fcc Modification of Solid Fullerene C60
Authors: Zubov V.I. , Tretiakov N.P. , Rabelo J.N. Teixeira , Ortiz J.F.Sanchez
Published: April 2011

We have applied the correlative method of unsymmetrized self-consistent field to study thermodynamic properties of the FCC phase of solid fullerene C60 which is stable at T≥261 K. We have calculated the temperature dependences of the intermolecular distance, cohesive energy, bulk moduli, thermal expansion coefficient, elastic constants and sound velocities. The central intermolecular potential of Girifalco and its approximation by Yakub have been used. We have infered the crucial role of anharmonicity of lattice vibrations at high temperatures. It has been noted also that unlike other Van der Waals crystals, for the considered modification of C60 the quantum effects are small at any temperature. Our results agree closely with available experimental data. Some properties near the spinodal point are discussed.
Vibrational and Electronic Properties of RbC60 Thin Films
Authors: Lopinski G.P. , Mitch M.G. , Fox J.R. , Lannin J.S.
Published: April 2011

In situ Raman scattering, UPS and HREELS measurements have been used to study the fcc to orthorhombic phase transition in RbC60 thin films. Large changes in the Raman spectra are interpreted in terms of increased interfullerene coupling in the orthorhombic phase. However, the data do not support the proposed polymer model for this phase. Photoemission measurements indicate only small differences in the electronic states between the two phases in contrast with photopolymerized C60 where additional states are observed. Low energy metallic-like excitations and screening of intramolecular vibrations are observed in HREELS.
Lattice-Type Polymers from an Adduct of C60 and 2-Methylaziridine
Authors: Nigam A. , Shekharam T. , Bharadwaj T. N. , Giovanola J. , Narang S. C. , Malhotra R.
Published: April 2011

A one to ten adduct of C60 and 2-methylaziridine was prepared and copolymerized with Novolac, Epon and Bisphenol A to obtain three dimensional polymers containing C60 with low coefficients of friction and good wear properties, which bode well for their potential use as solid lubricants.
Embedding Fullerenes in Thin Sol-Gel Films
Authors: Prato Maurizio , Maggini Michele , Scorrano Gianfranco , Brusatin Giovanna , Innocenzi Plinio , Guglielmi Massimo , Meneghetii Moreno , Bozio Renato
Published: April 2011

Fullerene C60 can be considered an interesting molecule for the preparation of new materials with attractive optical properties. In the present work we report the preparation of sol-gel derived, glass-like materials containing C60 and a C60 derivative prepared by 1,3-dipolar cycloadditions of azomethine ylides to C60. The optical characterisation of the films confirms the presence of C60 and its derivative, in the glass matrix.
Disilene Addition to C70
Authors: Jacobs S. J. , Rohlfing C. M. , Cahill P. A.
Published: April 2011

Semiempirical and ab initio Hartree Fock computational results indicate that the highly reactive dienophile tetramethyldisilene, Me2Si=SiMe2, is an excellent candidate for a novel functionalization of the equator of C70 via a [2+4] cycloaddition to the 21, 22, 23, 42 carbons. Thermal or photochemical generation of tetramethyldisilene in the presence of C70 results in similar complex mixtures in which the major product appears to be that of [2+2] cycloaddition to the 7,8 carbons of C70. A minor product clearly results from [2+2] cycloaddition to the 1,9 carbons. Both of these products are hydrolytically unstable and are converted nonspecifically to mixtures of 1,9- and 7,8-C70H2 which are also present in HPLC traces of the reaction mixtures.
Fullerenes as Nanoscale “Connectors”
Authors: Meier Mark S. , Rice Douglas J. , Thomas Craig , Majidi Vahid , Pogue Robert , Poplawska Magdalena
Published: April 2011

The construction of nanoscale devices is a potentially important area of technology. Efforts in this area of chemistry have largely concentrated on the preparation of rigid organic molecules for use as spacers. These compounds are used to separate electronically active units, usually metal complexes, by defined distances.

We have been investigating the use of fullerenes as connectors in the construction of these extended assemblies. In this paper we will present our results on the preparation of these compounds, including discussions of the regiochemistry of the second addition of a nitrile oxide to a fullerene isoxazoline.
Heteroepitaxial Growth of Epitaxial C60-Thin Films on Mica(001)
Authors: Henke S. , Thürer K.H. , Geier S. , Rauschenbach B. , Stritzker B.
Published: April 2011

On mica(001) thin C60-films are deposited by thermal evaporation at substrate temperatures from room temperature up to 225°C. The dependence of the structure and the epitaxial alignment of the thin C60-films on mica(001) on the substrate temperature and the film thickness up to 1.3 μm at a well-defined deposition rate (0.008 nm/s) is investigated by atomic force microscopy and X-ray diffraction. The shape and the size of the C60-islands, which have an influence on the film quality at larger film thicknesses, are sensitively dependent on the substrate temperature. At a film thickness of 200 nm the increase of the substrate temperature up to 225°C leads to smooth, completely coalesced epitaxial C60-thin films characterized by a roughness smaller than 1.5 nm, a mosaic spread Δω of 0.1° and an azimuthal alignment ΔΦ of 0.45°.
Low Temperature Formation of β-SiC by C60Deposition on Silicon
Authors: Henke S. , Rauschenbach B. , Stritzker B.
Published: April 2011

By deposition of C60 on silicon at moderate temperatures (800°C … 900°C) the formation of thin epitaxial β-SiC-films on Si could be proved. C60 -molecules were deposited onto Si(001) and Si(111) in high-vacuum at constant deposition rates for some hours. The thickness and the composition of the formed layers are determined by Rutherford-Backscattering (RBS). The thickness of the layers varied between about 50nm and 200nm in dependence of the deposition parameters. From the shape of the RBS-spectra only β-SiC can be identified. SiC-grains with a mean size of about 500 nm have been observed by atomic force microscopy (AFM). X-ray diffraction (XRD) pole figure measurements demonstrate the heteroepitaxial growth of β-SiC on Si It can be shown by XRD that only the cubic structure (β-SiC) of the different polytypes of SiC was formed during the carbonization process. The formation of growth defects (twins) can be observed.
Nano and micro structure of laser damaged fullerite single crystal
Authors: Manfredini M. , Milani P.
Published: April 2011

We have studied the nano and micro structure of the product resulting from high fluence laser irradiation of C60 single crystal. Raman spectra reveal that, in the presence of oxygen, amorphous carbon (a-C) is formed. The laser heating of the sample promotes fullerene o:ddation and cage opening reactions: the new carbon phase is originated by the coalescence of C60 fragments. The irradiance threshold tbr a-C tbrmation is discussed. Brillouin spectroscopy shows that the acoustic properties of the irradiated material are typical ofa higly porous material similar to carbon aerogels. Under irradiation in an inert environment C60 degradation is not observed, even for high power densities.
Morphology of Nanometric Boron Nitride Powders Produced by Laser Pyrolysis
Authors: Willaime F. , Boulanger L. , Cauchetier M.
Published: April 2011

Ultrafine boron nitride powders were synthesized by laser driven reactions in BC13-NH3 mixtures. The structure and morphology of the graphitic nanoparticles generated in this process were investigated by high-resolution electron microscopy. Polyhedral concentric shells (ranging in size from 30 nm to more than 100 nm) are a major constituent of the as-pyrolyzed powder. This onion-like configuration is very similar to that observed in carbon materials. After heat treatment at 1650°C under nitrogen atmosphere, plate-like particles with a few perfectly flat graphitic sheets (10 to 50 layer thick, 50 nm in diameter) are formed.
Characterization of C60 Fullerene in Carbon Disulfide Solvent Using Intermediate Angle Neutron Scattering
Authors: Spooner Steve , Zarestky J. L. , Affholter K. A.
Published: April 2011

In recent characterization of fullerenes by small angle neutron scattering use was made of the Guinier scattering regime to determine the radius of gyration of the fullerene molecules. In the present work the HB-1A spectrometer at the High Flux Isotope Reactor at Oak Ridge National Laboratory was used to measure the scattering from C60 in CS2 solvent. The scattering covers a range which includes scattering intensity oscillations associated with the C60 shell structure. These oscillations are used to measure the C60 molecule size. Assumption of a carbon shell radius of 3.57Å fits the scattering minima. This radius and the assumption of a solvent cavity radius of 5.93 Å gives a good fit to the whole scattering range. However, modeling to remove a remaining scattering intensity discrepancy at low angles requires the addition of a small solvent density excess near the C60 molecule.
The Binary Phase Diagram Naphthalene-C60
Authors: Hidalgo-Quesada Rafael , Zhang Xin-Yu , Giessen Bill C.
Published: April 2011

Molten naphthalene has considerable solubility for C60 (∼5 10−3 in mole fraction) making it a potential solvent for fullerenes; the phase diagram is therefore important. It was also of interest to study whether C60 displays the same anomalous solubility behavior in naphthalene as in hexane, CS2, and toluene, where the solubility increases with temperature up to a maximum near room temperature and decreases subsequently.

The naphthalene-C60 eutectic temperature was determined by DSC; the solubility of C60 in the liquid solution up to ∼180 °C was derived by UV spectrophotometry and found to decrease monotonically with increasing temperature from a mole fraction of 5 · 10−3 at the eutectic (∼79.5 °C) to 1.5 · 10−3 at 165 °C; thus, it shows a higher-temperature C60 solubility decrease (with increasing temperature) analogous to the other solutions, while a potential lowertemperature solubility increase (with increasing temperature) is masked by the eutectic. A binary phase diagram based on these data is proposed.
Field Emission and Growth of Fullerene Nanotubes
Authors: Rinzler Andrew G. , Hafner Jason H. , Colbert Daniel T. , Smalley Richard E.
Published: April 2011

Efforts to control the growth of individual carbon nanotubes from nanotube seed crystals have led to a characterization of their field-induced electron emission behavior. The application of a bias voltage in our growth apparatus was motivated by the prolific formation of nanotubes in the carbon arc growth method, in which the electric field appears to play a central role. We report here the ability to achieve various tube tip configurations by the controlled application of voltage, heat and chemicals to an individual nanotube, and that these states are well characterized by the emission currents they induce.
MOCVD of Ferroelectric Pb(Zr,Ti)O3 and (Pb,La)(Zr,Ti)O3 Thin Films for Memory Device Applications
Authors: Shimizu Masaru , Shiosaki Tadashi
Published: April 2011

The advantages of MOCVD for the growth of Pb(Zr,Ti)O3 (PZT) and (Pb,La)(Zr,Ti)O3(PLZT) thin films were discussed, with emphasis on the controllability of film composition, crystalline structure and electrical properties. The possibilities of lowering the processing temperature and scaling up the process to commercial-based production were investigated. The preparation of PZT films and electrodes with a specific focus on improving the I-V and fatigue characteristics was also investigated.
Effects of Rheological Characteristics of Sol on Preferential Orientation of Pb(Mg,Zn)1/3Nb2/3O3 Thin Films
Authors: Jang Hyun M. , Cho Mun K.
Published: April 2011

Thin films of Pb(Mg,Zn)1/3Nb2/3O3 were fabricated by spin casting the Pb-Mg-Zn-Nb-0 complex alkoxide sols on (111)Pt-coated MgO (100) planes. It was observed that the rheological characteristics of sol greatly influenced the orientation of perovskite grains after thin-film formation. A strong preferential orientation of (100)-type planes of the perovskite grains was obtained in the thin films derived from the sols exhibiting pseudoplastic behavior. Small angle X-ray scattering experiment in the Porod region was performed to correlate the observed preferential orientation with the network structure of precursors at various stages of aging. It was shown that weakly branched precursor chains led to highly oriented grains after thin-film formation.
Preparation and Characterization of PMN (PbMgxNb1−xOz) Films by Sputtering
Authors: Cross J.S. , Tsukada M. , Kurihara K. , Kamehara N. , Niwa K.
Published: April 2011

Thin films of Pb(MgxNb1−x)Oz were produced by reactive sputtering from a lead target and columbite magnesium niobate powder targets at temperatures of 500–700°C on Pt/Ti/Si, Pt/SiO2/Si, MgO(lOO) and SrTiO3(100) substrates. The films in general deposited on Pt consisted of PbO and pyrochlore phases depending upon the deposition temperature. ICP analysis revealed that the films contained slightly less Mg than the target. This was attributed to negative ion preferential resputtering of the film within the discharge. However, films deposited at 650°C on MgO and SrTiO3 substrates from a lead target and magnesium niobate target which contained excess MgO, contained a highly oriented perovskite (100) Pb(Mg1/3Nb2/3)O3 [PMN] phase according to X-ray diffraction analysis. It was observed that the substrate composition and orientation greatly influenced the crystallinity of the deposited films.
SrxBa(1−x)TiO3 Thin Films for Active Microwave Applications
Authors: Horwitz J.S. , Pond J.M. , Tadayan B. , Auyeung R.C.Y. , Dorsey P.C. , Chrisey D.B. , Qadri S.B. , Muller C.
Published: April 2011

The dielectric constant, loss tangent and Curie temperature for SrxBa(1−x)TiO3 (SBT) thin films with x = 0.2 – 0.8 have been investigated at microwave frequencies. SBT films (0.5 -3 μm thick) were grown on (100) MgO and LaAlO3 substrates by pulsed laser deposition at substrate temperatures from 850 – 900 °C in 0.35 Torr of oxygen. Deposited ferroelectric films were single phase, highly oriented, and characterized by x-ray rocking curve widths of < 0.5°. Highly oriented SBT films with x-ray rocking curve widths of 72 arc seconds were observed. In general, the thin film dielectric constant at microwave frequencies is low (200–950) compared to the reported bulk value, but strongly dependent on the Sr/Ba ratio. Biasing of a ferroelectric interdigital capacitor (< 200 kV/cm) produces a change in the dielectric constant which resulted in a phase shift in the reflected signal (S11) measured as a function of frequency from 100 MHz to 10 GHz. The dielectric loss tangent measurement, as measured at room temperature and 9.2 GHz, ranges from 0.1 to 1.2 × 10−3 and depends on the Sr/Ba ratio. These data show that SBT thin films are suitable for the development of frequency tunable microwave circuits and components.
Pulsed Laser Deposition of Ferroelectric Multilayer Capacitors YBa2Cu3O7−δ/BaTiO3/YBa2Cu3O7−δ on Si(100) Substrates
Authors: Luo J. , Li Q. , Fenner D.B. , Liu S. , Hamblen W.D. , Wu X.D.
Published: April 2011

Ferroelectric multilayer capacitors (YBCO/BTO/YBCO) are fabricated on YSZ buffered Si (100) substrates by using pulsed-laser deposition technique. The deposition process is monitored in situ by using a Fourier Transform Infrared Spectrometer (FTIR), which measures the film's deposition rate and dielectric constant during the growth process. All four oxide layers in such a multilayer capacitor are grown in situ without breaking the vacuum, and without lowering the substrate temperature to below 600°C. The electrical properties and microstructures of the multilayer capacitor are studied and the optimal deposition condition are identified. Sharp resistive transitions are achieved for both the top and the bottom YBCO layers with on-sets at 90 K, zero resistance at 89 K for the bottom layer and 88K for the top layer. X-ray diffraction θ-2θand φ scans indicate that all four deposited oxide layers have their c-axis perpendicular to the substrate, and have less that 10% in-plane misorientation. The dielectric constant of the BTO layer is over 210. Hysteresis P-E characteristics and current-voltage curves are also measured to determine dielectric properties of the barium titanate films. Preliminary results with LaSrCoO (LSCO) electrode layer are also presented.
Pulsed Excimer Laser Deposition and Rapid Thermal Annealing of Ferroelectric Pb(Zr0.52Ti0.48)O3 Thin Films on Si-on-Insulator
Authors: Chen Yiqing , Zheng Lirong , Lin Chenglu , Xu Xinglong , Zou Shichang
Published: April 2011

Stoichiometric films of Pb(Zr0.52Ti0.48)O3 (PZT) were successfully deposited on Si-on-Insulator (SOI) substrates, with and without a platinum electrode, by an ArF (λ=193nm) pulsed excimer laser. Rapid thermal annealing (RTA) was used to transform amorphous or pyrochlore phase into ferroelectric perovskite phase. The film structure, composition, morphology, interface and electrical properties were studied by x-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (XTEM) and Sawyer-Tower circuit, respectively. It was found that there was a narrow operational window of annealing time for PZT films at fixed annealing temperature. Pure ferroelectric perovskite phase with mainly (100) and (110) orientations could be obtained in PZT films on Pt coated SOI substrates. The interfaces between the PZT films and both Si and Pt//Si were very abrupt, indicating there was no interdiffusion between them. Ferroelectric hysteresis loops showed a remanent polarization of 15μC/cm2 and a coercive field of 50KV/cm.
Pulsed Laser Deposition of Epitaxial LiNbO3 Films on Sapphire Substrates Using a Single Crystal Targets
Authors: Lee See-Hyung , Noh Tae W. , Lee Jai-Hyung , Kim Young-Gi
Published: April 2011

Pulsed laser deposition was used to grow epitaxial LiNbO3 films on sapphire(0001) substrates with a single crystal LiNbO3 target. Using deposition temperatures below 450 °C, LiNbO3 films with correct stoichiometry could be grown without using Li-rich targets. Rutherford backscattering spectrometry measurements showed that the oxygen to niobium ratio is 3.00 ± 0.15 to 1.00. It was also found that the crystallographic orientations of the LiNbO3 films could be controlled by adjusting the oxygen pressure during deposition. An x-ray pole figure shows that epitaxial LiNbO3 films were grown on sapphire(0001), but with twin boundaries.
Reactive Ion Etching Damage to Ferroelectric Thin Films Capacitors
Authors: Pan W. , Thio C.L. , Desu S.B. , Chung Cheewon
Published: April 2011

Reactive ion etching damage to sputtered Pt/PZT/Pt ferroelectric capacitors was studied using Ar and CHCIFCF3 etch gases. Electrical properties, hysteresis, fatigue, and leakage current of PZT capacitors, before and after etching, were compared to examine the etching damage. It is found that the damage effects depend on etching time and are mainly due to the physical bombardment effect. The PZT capacitors etched with CHCIFCF3 plasma showed less damage than those etched in Ar plasma. The electric properties of etched Pt/PZT/Pt capacitors are recovered by annealing at 400 °C for 30min.
Processing and Oxidation of Nitride Stabilized 18Cr-20Ni Stainless Steel
Authors: McDowell C. S. , Basu S. N. , Ando T.
Published: April 2011

The present investigation explores the use of nitride precipitates for grain-size stabilization in a 18Cr-2ONi austenitic stainless steel for high temperature applications in oxidizing atmospheres. The potential for reductions in alloying contents of such steels has become a focus of research out of concern for the conservation of strategic alloying elements such as Cr. In order for the alloy to maintain superior oxidation resistance, it must sustain a fine grain size by virtue of effective grain boundary pinning by stable precipitates.

Al and Ti microalloyed 18Cr-20Ni steels have been nitrided using rapid solidification processing (spray forming) and subjected to thermomechanical treatments to precipitate nitrides from the supersaturated solid solutions. The spray formed deposits were cold rolled to increase the density of dislocations, which act as sites for nucleation of nitride precipitates. The alloys were then annealed to nucleate and grow the precipitates, followed by cold rolling and recrystallization to generate a stable, fine grained microstructure. The Ti-microalloyed steel demonstrated high-temperature grain size stability by maintaining a grain size of≈7μm after 600h at 1000°C.

The Ti-microalloyed steel has been subjected to isothermal oxidation in air at 900°C. The results of preliminary oxidation experiments are reported. The spray deposited and thermomechanically processed alloy demonstrated high-temperature oxidation behavior superior to conventionally processed (hot rolled and annealed) alloys.
Mechanical Behavior of Energetic Materials During High Acceleration in An Ultracentrifuge
Authors: Lanzerotti M. Yvonne D. , Sharma J.
Published: April 2011

The mechanical behavior of explosives subjected to high acceleration (high g) has been studied in an ultracentrifuge. The experiments reported here reveal new information on the mechanical behavior of such materials and the influence of grain size on the fracture process. Through measurement and analysis of fracture surfaces, we have found that predominately intergranular failure occurs when the shear or tensile strength of the explosive is exceeded. We have found that the mechanical strength of melt-cast polycrystalline TNT varies inversely with crystal size. That is, if the sample consists of large, homogeneous crystals, these are found to separate from the sample at lower g-levels due to the larger mass-to-binding surface area ratio of the crystals. Conversely, smaller original crystallites are found to separate at higher g-levets due to the smaller mass-to-binding surface area ratio. Our results show that single crystals of TNT fracture under a higher g-level at crystal defects. We have also found that the fracture acceleration of Octol decreases with increasing percent TNT and decreasing percent HMX. Hexanitrostilbene (HNS) has been shown by other investigators to be an effective additive to prevent growth of large TNT grains. We have found that the fracture acceleration increases when HNS is added to Octol.
Synthesis of Nickel and Iron Ultrafine Powders and Mechanical Properties of Their Compacts
Authors: Alymov M. I. , Leontieva O. N. , Myshilyaev M. M.
Published: April 2011

Nickel and iron ultraflne powders have been produced by the method of hydrogen reduction of the low-water hydroxides obtained by geterogeneous interaction between solid salt and alkaline solution. Suggested method allows to obtain ultrafine powders with narrow particle size distribution and low impurity contents. Bulk samples based on nickel and iron ultrafine powders were produced by hot Isostatic pressing and high-temperature gas extrusion methods.
Mass Transport Through Polycrystalline Microstructures
Authors: Swiler T. P. , Holm E. A. , Young M. F. , Wright Steven A.
Published: April 2011

Mass transport properties are important in polycrystalline materials used as protective films. Traditionally, such properties have been studied by examining model polycrystalline structures, such as a regular array of straight grain boundaries. However, these models do not account for a number of features of real grain ensembles, including the grain size distribution and variations in grain shape. In this study, a finite difference scheme is developed to study transient and steady-state mass transport through realistic two dimensional polycrystalline microstructures. Comparisons with the transport properties of traditional model microstructures provide regimes of applicability of such models. The effects of microstructural parameters such as average grain size are examined.
Reaction Rate of Liquid-Quenchied Water-Floating Alloy
Authors: Aida T. , Nishi Y.
Published: April 2011

We investigate a reaction rate of Mg-Li Al alloy (specific gravity is 0.95) in water. The hydrogen gas is mainly generated as follows : Li + H2O → LiOH + 1/2 H2 ↑. Using the volume of hydrogen gas, the reaction rate is estimated. The apparent activation energy (E) is 4.63 kcal/mol for the early reaction period. It is approximately equal to that of the water diffusion. On the other hand, the E is 9 kcal/mol after the early reaction period at the 2 ml (the volume of hydrogen gas). Furthermore, we succeed to prevent the sample against the reaction by the use of coating paints.