bluuebunny/crossref_metadata_2025 huggingface dataset:
Auger Electron Spectroscopy of Metallic Film/Laser-Irradiated Alumina Couples
Authors: Park Jaewon , Pedraza Anthony J. , Allen William R.
Published: April 2011

Auger Electron Spectroscopy (AES) was employed to study metal/ceramic interfaces. Adhesion pull testing showed that a strong bonding between metallic films and alumina substrates is obtained when alumina is laser-irradiated in an oxygen atmosphere before deposition and, after deposition, the couple is annealed at 300°C for 1 h. On the other hand the gold/alumina bonding is extremely weak when laser-irradiation of the substrate is performed in argon-4% H2. Fresh surfaces were exposed for AES at several distances from the metal/ceramic interface by sputtering the specimens in situ after each analysis. In the region of the interface the sputtering rate was slower than in the film in order to minimize any spurious effect due to ion beam bombardment. AES of a gold/alumina couple prepared in this way reveals that the Auger peaks of gold from the film and oxygen and aluminum from the alumina substrate, when the substrate was irradiated in oxygen atmosphere, shift by ∼ 1.5 eV, 1.6eV, and 1.4eV respectively when the analysis area encompasses the metal/ceramic interface. On the other hand, no shift of the Auger peaks are observed at the interface when the laser-irradiation was performed in argon-4%H2. In the case of copper deposited on alumina laser-irradiated in an oxygen atmosphere, an interfacial compound is formed. This compound is a double oxide of aluminum/copper or two separated oxides, and promotes strong copper/alumina bonding.
Band Offsets of InAsxP1−X/InP Strained Layer Quantum Wells Grown by Lp-Movpe Using TBAs
Authors: Beaudoin M. , Masut R.A. , Isnard L. , Desjardins P. , Bensaada A. , L'Espérance G. , Leonelli R.
Published: April 2011

Low temperature optical absorption spectra are presented for a series of InAsxP1-x/InP strained layer multiple quantum well structures (0 < x = 0.35) grown by low pressure metal organic vapor phase epitaxy (LP-MOVPE) using trimethylindium (TMIn), tertiarybutylarsine (TBAs) and phosphine as precursors. The well widths and compositions in these structures are determined from high resolution x-ray diffraction and transmission electron microscopy. The absorption spectra are then analyzed by fitting the excitonic peak energy position with transition energies determined from a solution to the Schrödinger equation. We used the envelope function formalism with the Kane bands as the basis wavefunctions and included corrections for non parabolicity. From these fits and elasticity theory, both the bandgap of unstrained InAsxP1-x and the band offsets of these heterostructures are deduced self-consistently. The conduction band offsets are found between 72% and 75% of the total strained bandgap differences.
Epitaxial Growth of Aluminum Nitride on Sapphire and Silicon
Authors: Dovidenko K. , Oktyabrsky S. , Narayan J. , Razeghi M.
Published: April 2011

The microstructural characteristics of wide band gap semiconductor, hexagonal A1N thin films on Si(100), (111), and sapphire (0001) and (10ī2) were studied by transmission electron microscopy (TEM) and x-ray diffraction. The films were grown by MOCVD from TMA1 + NH3 + N2 gas mixtures. Different degrees of film crystallinity were observed for films grown on α-A12O3 and Si substrates in different orientations. The epitaxial growth of high quality single crystalline A1N film on (0001) α-Al2O3 was demonstrated with a dislocation density of about 2*10 10cm−2 . The films on Si(111) and Si(100) substrates were textured with the c-axis of A1N being perpendicular to the substrate surface.
Possible Mechanisms for Photoluminescence in Spark-Processed Si
Authors: Hummel R.E. , Ludwig M.H. , Chang S.-S.
Published: April 2011

Possible mechanisms which may explain the strong blue/violet (410 nm) photoluminescence (PL) in spark-processed Si (sp-Si) are critically reviewed and discussed. They include involvement of silicon-oxides, silicon-hydrides, silanol groups, siloxene, and silicon carbide. It is concluded from the experimental evidence that none of the above mechanisms can satisfactorily explain the major contribution(s) to PL in sp-Si.
Lattice Relaxation Effects in Si and GaAs Nanocrystals
Authors: Zhao X. S. , Ge Y. R. , Schroeder J. , Persans P. D.
Published: April 2011

Raman scattering results on porous silicon, and silicon and gallium arsenide nanocrystals show that almost all vibrational modes become Raman active and remarkably soft in these nanocrystal systems. The experimental results further demonstrate that the carrier-induced strain effects play an important role on the optical properties of such nanocrystal systems.
Nonlinear Optical Properties of Silicon Nanocrystallites: Effects of Passivation
Authors: Seraphin A.A. , Aranda F.J. , Werwa E. , Rao D.V.G.L.N. , Kolenbrander K.D.
Published: April 2011

Degenerate four-wave mixing with picosecond pulses at 532 nm has been used to study the third-order nonlinear optical susceptibility (x(3)) for a series of passivated thin films of silicon nanocrystallites. A pulsed laser ablation supersonic expansion source of isolated silicon nanocrystallites was used to deposit thin films onto inert substrates. These films were subsequently passivated using chemical etches or oxidation steps. We observe a strong dependence of the measured x(3) as a function of the degree of passivation, indicating the fundamental importance of the surface of the nanocrystallite in enabling the nonlinear optical behavior. Systems providing more complete passivation were found to have greatly enhanced X(3) behavior when compared to poorly passivated systems. Surface passivation is also shown to be critical to the visible photoluminescence behavior of the thin films, as poorly passivated nanocrystallites exhibit very weak light emission, while well passivated systems show efficient emission. In both cases, the passivation controls the recombination pathways of excited carriers and determines the material's optical properties.
X-Ray Absorption Spectroscopy and Optical Absorption Studies of the Growth of CdS Nanocrystals in Glass
Authors: Persans P. D. , Lurio L. B. , Pant J. , Olsson R. J. , Yukselici H. , Hayes T. M.
Published: April 2011

We have measured and analyzed optical absorption, Raman scattering and x-ray absorption spectra of CdS nanocrystals grown by solid-state reaction in a borosilicate glass. We observe the appearance and growth of an optical absorption peak which is attributed to CdS nanoparticles. Using x-ray absorption we quantify the progressive replacement of O nearest neighbors of Cd by S with heat treatment and the appearance and growth of a well-defined second nearest neighbor peak.
Calculation of the Electronic Structure of Silicon Nanocrystals
Authors: Hill Nicola A. , Whaley K. Birgitta
Published: April 2011

The densities of states for Si nanocrystals with diameters between 15 and 35 Å are calculated using a time-dependent algorithm within the tight-binding approximation [1]. The calculated effects of surface termination and surface relaxation on the electronic properties of Si are shown. The variation in band-gap with cluster size is calculated for clusters up to 60 Å in diameter.
Morphology-Dependent Spectroelectrochemical Behavior of PbS Nanoparticulate Films Grown Under Surfactant Monolayers
Authors: Tian Yongchi , Wu Changjun , Kotov Nicholas , Fendler Janos H.
Published: April 2011

Marked morphology-dependent electrical, electrochemical, and spectroelectrical properties of PbS nanostructured particulate films are the subject of the present communication. Examination of the potential-dependent absorption spectra on optically transparent conducting glass supports and of the voltage-dependent capacitances and photocurrents on platinum support have revealed substantial differences between equilateral-triangular PbS (PbS-I), right-angle-triangular PbS (PbS-II), and disk-shaped PbS (PbS-III) nanoparticulate films.
Luminescence and Resonance Raman Spectroscopy of Indirect Excitons in AgBr Nanocrystals
Authors: Pawlik S. , Stolz H. , Von Der Osten W.
Published: April 2011

AgBr nanocrystals with radii R ≈ 2.5 to 5 nm comparable with the exciton Bohr radius are produced in inverse micelles. As compared with bulk AgBr, the exciton emission exhibits a substantial blue shift and enhanced intensity due to the spatial confinement. Besides luminescence, first- and second-order resonance Raman scattering is discovered the occurrence of a zero-phonon process clearly revealing mixing of L with Г point states. From time-resolved measurements, exciton lifetimes of the order of 500 μs are found. They are close to the radiative lifetime in the bulk demonstrating that nonradiative processes are negligible in these nanocrystals. In agreement with state mixing, the total decay rate is found proportional to R−2. A small shift of the TO(L) Raman line with excitation photon energy is analyzed in terms of the wavevector dependent interaction of the quantized exciton states with the dispersive phonon.
First-Principles Calculation of the Optical Properties of Nanocrystalline Silicon
Authors: Hirao Masahiko
Published: April 2011

The electronic structure and optical properties of nanocrystalline silicon were calculated by the first-principles density functional pseudopotential approach. The calculated energy-gap upshift from the bulk-Si value is nearly proportional to the reciprocal of the crystallite size. Dipole transitions across the gap are weakly allowed and the transition elements decrease rapidly with increases in the crystallite size. The apparent lifetime, the time over which the intensity decreases to 1/e of the initial value, decreases sharply from milliseconds to microseconds within a certain temperature range. The effect of dehydrogenation and the structural stability were investigated using an ab initio molecular dynamics technique. When some of the surface hydrogen atoms are removed, subsequent lattice relaxation eliminates dangling bonds. Further dehydrogenation creates mid-gap states due to surface dangling bonds, which act as nonradiative recombination centers. The calculated results are compared with observations of porous Si.
Comparison of Porous Silicon Etched Gently and Under Illumination
Authors: Filios Adam A. , Tsu Raphael
Published: April 2011

Porous silicon samples prepared in the dark under "gentle" etching conditions clearly demonstrate effects of quantum confinement, such as a correlation of the photoluminescence peak energy with the downshift of the Raman line from 521 cm−1 for bulk silicon, and a blue shift in the remaining weak photoluminescence after thermal annealing. On the other hand, samples prepared under illumination as well as those heavily etched in the dark, though luminesce brightly, show no significant effects of quantum confinement, suggesting a different dominant mechanism for the observed luminescence.
Post-Anodization Implantation and CVD Techniques for Passivation of Porous Silicon
Authors: Duttagupta S. P. , Tsybeskov L. , Fauchet P. M. , Ettedgui E. , Gao Y.
Published: April 2011

Proper surface passivation is critical for achieving stable, efficient PL from light-emitting porous silicon (LEPSi). As-anodized LEPSi is passivated by hydrogen which desorbs at a temperature as low as 400 °C. For device purposes, it is necessary that porous Si can tolerate at least 450 °C for post anodization annealing/metallization steps. We have established that, if the hydrogen at the surface is substituted by oxygen, the resulting Si-Ox passivation is significantly more stable. One way of achieving this is to implant low energy/low dose oxygen to form a thin coating of SiO2 on the surface. Post implantation FTIR data report the absence of Si-H peaks. XPS data indicate the formation of nearly stoichiometric SiO2 at the surface. Similar results were achieved by implanting with nitrogen to form Si3N4. As an alternative to implantation, we have deposited thin capping layers of SiO2, Si3N4 and SiC by plasma-enhanced chemical vapor deposition (PECVD) which resulted in a similar degree of passivation. Wafers were pre-treated at 400 °C to remove hydrogen from the surface. Finally, we carried out a low-pressure CVD (LPCVD) oxide deposition on LEPSi. Post implantation/CVD annealing was done at temperatures up to 600 °C. In most cases, little or no change was observed in the resultant PL intensities.
Reciprocal Space Analysis of the Microstructure of Luminescent and Nonluminescent Porous Silicon Films
Authors: Lee S.R. , Barbour J.C. , Medernach J.W. , Stevenson J.O. , Custer J.S.
Published: April 2011

The microstructure of anodically prepared porous silicon films was determined using a novel x-ray diffraction technique. This technique uses double-crystal diffractometry combined with position-sensitive x-ray detection to efficiently and quantitatively image the reciprocal space structure of crystalline materials. Reciprocal space analysis of newly prepared, as well as aged, p+porous silicon films showed that these films exhibit a very broad range of crystallinity. This material appears to range in structure from a strained, single-crystal, sponge-like material exhibiting long-range coherency to isolated, dilated nanocrystals embedded in an amorphous matrix. Reciprocal space analysis of n+and p+porous silicon showed these materials are strained single-crystals with a spatially-correlated array of vertical pores. The vertical pores in these crystals may be surrounded by nanoporous or nanocrystalline domains as small as a few nm in size which produce diffuse diffraction indicating their presence. The photoluminescence of these films was examined using 488 nm Ar laser excitation in order to search for possible correlations between photoluminescent intensity and crystalline microstructure.
Dimensions of Luminescent Porous Silicon by Thermal Effusion of Hydrogen
Authors: Nikolov A. , Petrova-koch V. , Polisski G. , Koch F.
Published: April 2011

Using electrochemically prepared porous Si with hydride covered internal surfaces, we determine the H-content by thermal effusion and measurement of the total gas evolution. The amount of Si is obtained by weighing. In this way, we derive a value of the ratio of H- to Si-atoms and from the latter a value of the linear dimension of the luminescing particles
Photoluminescence Quenching of Porous Silicon
Authors: Fisher Diana L. , Gamboa Aurilee , Harper Jessica , Lauerhaas Jeffrey M. , Sailor Michael J.
Published: April 2011

Photoluminescence from porous silicon can be quenched reversibly by a variety of molecular species. Quenching pathways for chemically incorporated surface species, physisorbed species (that undergo no net chemical transformation), and for electron donating reagents have all been identified. For systems involving charge transfer quenching, the concentration dependence typically follows a Stern-Volmer type of relationship, with the more easily oxidized molecules producing the largest Stern-Volmer slopes (most efficient quenching). The slope of the Stern-Volmer plot is dependent upon the particular wavelength that the Stern-Volmer data are obtained from. Data interpretation is complicated by the fact that porous Si shows a wavelength dependent emission lifetime, although when this is taken into account the data can be qualitatively described within the context of a driving-force dependent quenching model. A simple model for understanding the various photoluminescence quenching phenomena observed with porous Si is presented, involving an ensemble of emissive states with energy dependent lifetimes and Stern-Volmer quenching behavior. The model adequately accounts for both red and blue spectral shifts that have been observed upon photoluminescence quenching.
Carrier Dynamics in Porous Silicon: from the Femtosecond to the Second
Authors: Fauchet Philippe M.
Published: April 2011

The luminescence in red-emitting porous silicon exhibits a distribution of lifetimes in the μsec time domain at room temperature and in the msec time domain at cryogenic temperatures. However, the luminescence and carrier dynamics in porous silicon display transients that vary from much less than 1 psec to ∼ 1 sec, depending on the measurement conditions and sample preparation. We have investigated the carrier dynamics in porous silicon by two time-resolved techniques. The blue photoluminescence of oxidized porous silicon has been measured with 100 ps time resolution as a function of the oxidation method, emission wavelength, excitation intensity and measurement temperature. The blue luminescence has a distinct origin from the well-studied red luminescence and we attribute it to defects in the oxide. Femtosecond photoinduced absorption measurements have been performed on thin red-emitting porous silicon films. The wavelength and intensity dependence of the recovery are interpreted in terms of trapping and of Auger recombination at high excitation intensity. Our results also show conclusively that red-emitting porous silicon is not a direct gap semiconductor.
Micron-Size and Submicron-Size Light-Emitting Porous Silicon Structures
Authors: Duttagupta S. P. , Fauchet P. M. , Peng C. , Kurinec S.K. , Hirschman K. , Blanton T. N.
Published: April 2011

We have developed three classes of techniques to produce micron-size and submicron-size light emitting porous Si (LEPSi) patterns and to protect the rest of the wafer. In the 1st class, LEPSi lines down to 2 µm width have been made using a photoresist/silicon nitride trilayer mask, followed by anodization. PL mapping of the structures indicates that the protected regions have not been etched. Using electron beam lithography sub-0.5 micron porous Si lines have been generated. In the 2nd class, formation of porous Si is inhibited by amorphizing Si using ion implantation followed by anodization and annealing. The crystallinity and electrical properties of the implanted region have been fully characterized after annealing. Using focussed ion-beam implantation, LEPSi patterns of the order of 100 nm have been obtained. The 3rd class consists of enhancing the formation of porous Si by a low energy/low dose bombardment (ion-milling) with argon ions prior to anodization. Under appropriate conditions, we have observed a strong enhancement of the formation rate of LEPSi where bombardment took place, possibly due to the generation of a large number of defects on the wafer surface.
Light Emission Versus Excitation from Porous Structures in Ion-Implanted Silicon
Authors: Vázsonyi É. , Barsony I. , Lohner T. , Fried M. , Erostyák J. , Rácz M. , Pászti F.
Published: April 2011

The quantum efficiency of electroluminescence (EL) in porous silicon structures (PSL) is practically limited by the mechanical instability of higher porosity layers. This can be overcome by adding a thin implanted p+ surface region before the formation of PSL. The resulting structure consisted of different morphologies depending on the local doping level. When the implantation doping level and diffusion depth was increased, reduced photoluminescence (PL) from the otherwise strongly emitting buried PSL of 80% porosity was obtained. Spectroscopic ellipsometry provided detailed analysis of optical transmission of PSL prepared on a boron implanted substrate. Since the spectral transmission of PSL is morphology dependent, in this novel structure the excitation wavelength can be absorbed in the upper mesoporous p+ region, which is much more transparent for the visible emission spectrum. The width and peak position as well as integral intensity of EL spectra during anodic oxidation of the novel, workable structure were not affected by the doping level in the thin surface region.
A Monte Carlo Simulation of the Stillinger-Weber Model for Si-Ge Alloys
Authors: Laradji Mohamed , Landau D. P. , Dünweg B.
Published: April 2011

The bulk phase behavior of silicon-germanium alloys is investigated by means of a constant pressure Monte Carlo simulation of the Stillinger-Weber potential in the semi-grand-canonical ensemble. At low temperatures, Si and Ge phase separate into a Si-rich phase and a Ge-rich phase. The two-phase region is terminated by a critical point whose nature is investigated thoroughly by the multihistogram method combined with finite size scaling analysis. These results showed that the critical behavior of the alloy belongs to the mean field universality class, presumably due to the elastic degrees of freedom. We have also studied the structural properties of the mixture and found that the linear thermal expansions of both Si and Ge agree well with experiments. We also verified Végard's law above the critical point and calculated bond length distributions.
Carrier Transport in Porous Silicon Light-Emitting Diodes
Authors: Peng C. , Fauchet P.M. , Hirschman K.D. , Kurinec S.K.
Published: April 2011

This work presents a systematic investigation of carrier injection and transport mechanisms in light emitting porous silicon (LEPSI). In addition to optical characterization of the LEPSI film, the electrical characteristics of LEPSI light emitting diodes (LEDs) and the device parameter effects on electroluminescence (EL) efficiency are explored including junction structure, LEPSI layer thickness, and chemical treatment. Au/LEPSI device structures exhibit a power-law current-voltage relationship (I∼Vm) and LEPSI pn junction device structures exhibit an exponential current-voltage relationship (I∼eeV/nkT). The carrier transport mechanisms are discussed, and theoretical simulations match well with the experimental data. An increase in EL efficiency has been achieved by adjusting the LEPSI layer thickness, implementing chemical treatment, and improving the quality of the contacts. The EL response to the frequency modulation of LEPSI LEDs was studied, resulting in a cutoff frequency in excess of 100 kHz. LEPSI LEDs have demonstrated stable EL at applied voltages as low as 5 volts with EL efficiencies of 0.01%.
Electroluminescence of Yb-Doped InP
Authors: Alshawa A. K. , Lozykowski H. J. , Brown I.
Published: April 2011

Detailed experimental and theoretical studies of the electroluminescence excitation mechanism of Yb3+ in InP are presented. The electroluminescence spectra and the kinetics of Yb implanted InP were investigated under pulsed and dc excitations at temperatures over the range 9 - 70 K. The electroluminescence spectrum of the 4f transitions 2F5/2 - 2F7/2,, at 9 K, consists of a peak at 1001 nm and broader peaks in the spectral range between 1003 nm and 1010 nm. The intensity and current versus voltage characteristics have been recorded. The emission intensity increased linearly with current and started to saturate at high currents. The plot of the natural logarithm of intensity versus V−1/2 shows a straight line characteristic over three orders of emission intensity, showing that the direct impact excitation mechanism is a dominant process. At low temperature, the rise and decay times of the 1001 nm emission line were 9.63 µs and 10.50 µs, respectively. The thermal quenching energy, EA, was found to be 100 meV for both the electroluminescence intensity and the decay time.
Preparation and Optical Properties of Ultrathin Silicon Films
Authors: Collins R. W. , Nguyen Hien V. , An Ilsin , Lu Yiwei , Wakagi M.
Published: April 2011

Ultrathin crystalline silicon (c-Si) and amorphous silicon (a-Si:H) films have been prepared using plasma-enhanced chemical vapor deposition (PECVD) and atomic hydrogen etching methods. The complex dielectric functions (2 < hv < 4.5 eV) of films consisting of isolated clusters have been measured in situ and in real time using spectroscopic ellipsometry. For 12 Å c-Si cluster films, a sharp absorption onset is observed near 3 eV that blue-shifts with decreasing thickness, in consistency with the quantum confinement of electrons. A much broader absorption onset, observed for ∼13 Å a-Si:H cluster films near 2 eV, is attributed to an electron mean free path that is less than the cluster size, which limits the appearance of confinement effects.
Observation of Quantum Size Dependent Blue Shift in the Luminescence of Recrystallized Si/SiNx Superlattices
Authors: Grützmacher D.A. , Steigmeier E.F. , Auderset H. , Morf R. , Delley B. , Wessicken R.
Published: April 2011

Light emitting Si nanostructures were fabricated by recrystallizing Si/SiNx superlattices grown by plasma enhanced chemical vapor deposition. After recrystallisation by a laser annealing or by rapid thermal annealing the Raman spectra show clearly a confinement effect. A substantial photoluminescence around 470 to 550 nm and at 620 nm is observed after a hydrogen passivation step. The photoluminescence at 620 nm is assigned to carrier recombination via efficient surface states, whereas the blue emission at 470 to 550 nm shows a behaviour expected for quantum confinement effects.
Carrier Transport in Polycrystalline and Amorphous Silicon Thin Film Transistors
Authors: Sameshima T. , Sekiya M. , Hara M. , Sano N. , Kohno A.
Published: April 2011

The technologies of laser crystallization and methods of SiO2 formation in remote plasma chemical vapor deposition or SiO evaporation with an oxygen ambient realize the fabrication of n-channel polycrystalline and amorphous silicon thin film transistors (poly-Si and a-Si TFTs) at a temperature lower than 300 °C. The defect density was achieved to be 2∼3×1011 cm−2eV−1 and threshold voltage was about IV for both TFTs. The maximum field effect mobility was 600 cm2/Vs for poly-Si TFTs and 2.6 cm2/Vs for a-Si TFTs. The mobility of poly-Si TFT decreased as the gate voltage increases. This is interpreted as that the electrons are confined in the narrow inversion layer and electron scattering with phonon is enhanced for higher normal electric field.
Formation of Highly-Uniform and Densely-Packed Arrays of GaAs Dots by Selective Epitaxy
Authors: Tsai Charles S. , Lee Robert B. , Vahala Kerry J.
Published: April 2011

Formation of highly-uniform and densely-packed arrays of GaAs dots by selective epitaxy using diethylgallium-chloride and arsine is reported. The arrays of GaAs dots are imaged using atomic force microscopy (AFM). Accounting for the AFM tip radius of curvature, the smallest GaAs dots formed are 15-20 nm in base diameter and 8-10 nm in height with slow-growth crystal planes limiting individual dot growth. Completely selective GaAs growth within dielectric-mask openings at these small size-scales is also demonstrated. The uniformity of the dots within each array ranged from 6% for the larger dots to 16% for the smallest dots (normalized standard deviations of the areas of individual dots within each array).
Determination of the Strain Status of GaAs/AlAs Quantum Wires and Quantum Dots
Authors: Darhuber A.A. , Bauer G. , Wang P.D. , Song Y.P. , Torres C.M. Sotomayor , Holland M.C.
Published: April 2011

We have investigated periodic arrays of 150 and 175 nm wide GaAs-AlAs quantum wires and quantum dots, fabricated by electron beam lithography and SiCI4/O2 reactive ion etching, by means of reciprocal space mapping using triple axis x-ray diffractometry (TAD). The reciprocal space maps reveal that after the fabrication process the lattice constant along the growth direction slightly increases for the wires and even more so for the dots.
Photoluminescence of a Single-Crystal Silicon Quantum Well
Authors: Saeta Peter N. , Gallagher Alan C.
Published: April 2011

Single crystal-silicon quantum well layers with SiO2 barriers were grown from silicon-on-insulator substrates. Photoluminescence in the red and near-infrared was observed for average layer thickness < 8 nm, with peak signal for 2-nm thickness. The luminescence spectrum was essentially independent of well width for SiO2 barriers, but the photoluminescence intensity decreased sharply after annealing in Ar. These results suggest the importance of radiation from surface states. In contrast to oxide-passivated silicon nanocrystals and to porous silicon, the room-temperature photoluminescence quantum efficiency is low (10-4-10-5), probably due to variations in layer thickness and to diffusion of photoexcited carriers to fast nonradiative recombination centers.
Molecular Form-Factor and Analysis of Diffraction Pattern of Fullerene Crystals
Authors: Shulakov E. V. , Dilanyan R. A. , Rybchenko O. G. , Shekhtman V. Sh.
Published: April 2011

C60 molecules, in diffraction terms, is a quite novel object- covered by scattering points large sphere (D≅0.5acub). The peculiarities of the diffraction distribution and its variation near the order-disorder phase transition are defined from the buckybal's molecular form-factor, fmol. The comparison of calculated diffraction distributions and experimental data proved advantage of the C60 “jump rotation” model at middle and high temperatures.
A Molecular Dynamics Investigation of C60 –Rare Gas Mixtures
Authors: Abramo Maria C. , Caccamo C.
Published: April 2011

Mixtures of krypton atoms and C60, modelled in terms of rigid spherical molecules, are investigated at T ≃ 1800K through classical Molecular Dynamics. The two species do not tend to mix for low to moderate concentrations of krypton, and solid clusters of C60 tend to be stable. Possible implications of these results are shortly discussed.
Molecular Dynamics Studies of Nanotube Growth in a Carbon ARC
Authors: Brabec C. J. , Maiti A. , Roland C. , Bernholc J.
Published: April 2011

It has been shown experimentally that the growth of carbon nanotubes in an arc discharge is open-ended. This is surprising, because dangling bonds at the end of open tubes make the closed tube geometry more favorable energetically. Recently, it has been proposed that the large electric fields present at the tip of tube is the critical factor that keeps the tube open. We have studied the effects of the electric field on the growth of the nanotubes via ab initio molecular dynamics simulations. Surprisingly, it is found that the electric field cannot play a significant role in keeping the tubes open, implying that some other mechanism must be important. Extensive studies of the energetics and simulations of the growth of tubes were performed using a threebody Tersoff-Brenner potential. Our results show that there exists a critical diameter of ∼ 3 nm above which a defect-free growth of a straight tubule is possible. Narrower tubes stabilize configurations with adjacent pentagons that lead to tube-closure and termination of the growth. This explains the absence of tube narrower than 2.2 nm in arc discharge experiments.
Superconductivity at 40K in Cesium Doped C60
Authors: Palstra T.T.M. , Zhou O. , Iwasa Y. , Sulewski P.E. , Fleming R.M. , Zegarski B.R.
Published: April 2011

We report superconductivity in Cs3 C60 at 40K using ac susceptibility measurements under hydrostatic conditions up to 15 kbar. Cs3 C60 was prepared by reaction of C60 with Cs in liquid ammonia, followed by heating at 150°C. This route circumvents formation of the energetically more stable CSl C60 and Cs4 C60 phases. We have studied the synthesis and phase formation by measuring the symmetric Ag pentagonal pinch mode of C60 using Raman spectroscopy. Whereas Raman spectroscopy indicates homogeneous charge transfer of three electrons, x-ray diffraction indicates two phases other than the commonly observed fcc structure. This superconducting transition temperature is considerably higher than for known doped C60 compounds and intermetallic compounds.
Formation of Nanoparticles in A Carbon Arc
Authors: Majetich S. A. , Scott J. H. , Brunsman E. M. , Mchenry M. E.
Published: April 2011

Details of the carbon-coated nanoparticle growth mechanism are revealed by a comparison of product morphology and reactor parameters. Carbon-coated metal or metal carbide clusters nucleate in the gas phase, grow to characteristic sizes, and deposit on surfaces within the reactor. The surface temperature determines the crystallinity of the nanoparticles and the surrounding carbon. Tungsten carbide nanoparticles show that the carbon coating arises due to phase segregation when the nanoparticle has a lower melting point than that of graphite.
Rb1C60: Linear Polymer Chains and Dimers
Authors: Martin Michael C. , Koller Daniel , Rosenberg A. , Kendziora C. , Mihaly L.
Published: April 2011

The infrared- and Raman-active vibrational modes of C60 were measured in the various structural states of Rb1C60. According to earlier studies, Rb1 C60 has an f cc structure at temperatures above ∼ 100°C, a linear chain polymer orthorhombic structure when slowly cooled, and an as yet undetermined structure when very rapidly cooled (“quenched”). We show that the spectra obtained in the polymer state are consistent with each C60 molecule having bonds to two diametrically opposite neighbors. In the quenched state, we find evidence for further symmetry breaking, implying a lower symmetry structure than the polymer state. The spectroscopic data of the quenched phase are shown to be consistent with Rb2(C60)2, a dimerization of C60.
Geometric and Electronic Structure of Fullerene Film Growth as a Function of Coverage
Authors: Reihl B.
Published: April 2011

We have employed scanning tunneling microscopy at room and low temperature, i.e. 300, 50, and 5 K, to study the epitaxy and growth of fullerene films on the noble-metal surfaces Ag(110) and Au(110). Initial island growth occurs on terrace sites away from substrate step edges. Particularly at low temperatures where the rotational and vibrational movements of the fullerene molecules are frozen in, different intra-molecular topographic patterns become visible in ordered films, which are characteristic of particular adsorption sites. Complementary tunneling spectroscopy and direct and inverse photoemission measurements reveal distinct differences between the first adsorbed monolayer and additional fullerene layers indicating differences in bonding and charge transfer. Our results are compared to theoretical calculations.
Structural Characterization of Fullerene thin Films Fabricated by Organic Molecular Beam Deposition
Authors: Yase Kiyoshi , Saraya Takuya , Kudo Kazuhiro
Published: April 2011

Fullerene C60 thin solid films have been fabricated on the (001) surfaces of alkali halides (NaCl, KCI and KBr) and muscovite by organic molecular beam deposition (OMBD) technique. Pure C60 powder was sublimated from the precisely temperature-controlled K-cell at low pressure (10−7 Torr) and deposited onto the substrates kept in the range 125 –300 °C. Increasing both the temperature of K-cell and the substrate temperature, fine crystals grow with different size, shape and distance between neighbors. The dependence of the size and density of island crystals on the deposition conditions revealed the mechanism of crystal growth and the effect of interaction between molecules and substrate surface.
The Effects of Mechanical Alloying of C60 with Metals (II): Al and Mg
Authors: Zhang Xin-Yu , Hidalgo Rafael Q. , Murphy Richard J. , Markiewicz Robert S. , Giessen Bill C.
Published: April 2011

C60 has been subjected to multiple micro-alloying passes by co-rolling together with Al and Mg. Complete amorphization has been found in samples of C60 with Al. However, the structural changes in the C60/Mg combination were more complex. Here, a chemical reaction occurred during which the C60 molecules were apparently broken up into amorphous carbon-like materials, while a Mg-containing oxidation reaction product formed.
Structure and Conductivity of Iodine-Doped C60 Thin Films
Authors: Chen Jun , Zhang Haiyan , Chen Baoqiong , Peng Shaoqi , Ke Ning , Wong S.P.
Published: April 2011

We report here the results of our study on the properties of iodine-doped C60 thin films by IR and optical absorption, X-ray diffraction, and electrical conductivity measurements. The results show that there is no apparent structural change in the iodine-doped samples at room temperature in comparison with that of the undoped films. However, in the electrical conductivity measurements, an increase of more that one order of magnitude in the room temperature conductivity has been observed in the iodine-doped samples. In addition, while the conductivity of the undoped films shows thermally activated temperature dependence, the conductivity of the iodine-doped films was found to be constant over a fairly wide temperature range (from 20°C to 70°C) exhibiting a metallic feature.
Intermolecular Vibrations in Fullerene Systems
Authors: Mitch M.G. , Lopinski G.P. , Lannin J.S.
Published: April 2011

Intermolecular vibrations in a variety of fullerene systems are reported that provide information on C60-C60 and alkali-metal - C60 interactions and disorder effects. Low frequency Raman scattering in laser polymerized C60 films are shown to exhibit additional modes not reported for bulk materials. This result is consistent with recent dimer calculations. Raman scattering and HREELS measurements are shown to provide complementary information on alkali-metal motions in K3C60. In FCC Na6C60 both octahedral site disorder effects and reduced Na-C bond lengths account for changes in the Raman spectrum relative to Na3C60.
Measurement of the Excited-State Molecular Polarizability of C60
Authors: Tang N. , Hellwarth R. W. , Partanen J. P.
Published: April 2011

We use ∼30 ps pulses at 532 nm to measure the complex excited-state molecular polarizability αe in a C60/benzene solution. We determine the imaginary part of αe by measuring the excited-state absorption cross-section in a pump-probe experiment. In a degenerate-four-wave-mixing (DFWM) experiment, we find that in delayed probing of the complex index gratings formed by ∼30 ps pulses, the thermal and the excited-state polarizability changes both contribute to these transient gratings.
Analysis of a C60/Twelve Solvent Survey for Red Shifting of Spectra
Authors: Brandelik D. M. , Mclean D. G. , Sutherland R. L.
Published: April 2011

A survey of solvents with a wide variation of solubility for C60 and a broad variation in dielectric constant was investigated for possible red shifting of the linear absorbance spectra. Initial analysis shows that very small shifts are achieved. Molar extinction spectra were measured for C60 in each solvent in both the standard mode and with an integrating sphere. Comparison of the two methods has shown strong scattering effects. The UV-vis spectra revealed some broadening in the bands and in some cases supression of strong C60 bands. Fluorescence spectra also show broadening of bands and new fluorescence minima in some solvents indicating formation of complexes between the C60 and those solvents that contain nitrogen.
Thermodynamics of Co-Existing Phases at Phase Transitions in Fullerenes
Authors: Stepanov Eugene V.
Published: April 2011

A thermodynamic model describing the co-existence of phases at orientational phase transition in solid fullerenes is developed. It is shown that for such a transition in fullerene C60 characterized by the comparatively low enthalpy, heterophase fluctuations can lead to the wide temperature region of the phase co-existence within about 30 K below the 260 K transition point, and mainly determine the anomalies of thermodynamic characteristics observed within that region. By comparison of the theoretical dependence obtained on this basis with the experimental data on the x-ray diffraction and heat capacity measurements for C60, the energetical characteristics of the heterophase cluster formation are evaluated. The relative contribution of homophase and heterophase fluctuations to breaking the orientational order is discussed.
Effects of Catalyst Promoters on the Growth of Single-Layer Carbon Nanotubes
Authors: Kiang Ching-Hwa , Goddard William A. , Beyers Robert , Salem Jesse R. , Bethune Donald S.
Published: April 2011

The discovery of a catalytic route to the growth of single-layer carbon nanotubes suggests that it may be possible to produce these materials with better selectivity and in higher yield. Increasing the production efficiency is essential for characterization and application of these materials. We have discovered several catalyst promoters, in particular S, Bi, and Pb, that greatly enhance the single-layer carbon nanotube yield, and extend the distribution of nanotube diameters to much larger sizes (> 3 nm). Co crystallites encapsulated in graphitic polyhedra also form abundantly when S, Bi, or W is present. Understanding these catalytic process is of substantial scientific and technological importance.
Nanobundles
Authors: Yase Kiyoshi , Tanigaki Nobutaka , Kyotani Mutsumasa , Yumura Motoo , Uchida Kunio , Ohshima Satoshi , Kuriki Yasunori , Ikazaki Fumikazu
Published: April 2011

Fine molecular straw, carbon nanotube (NT) is aligned to form a bundle. The purified NTs are mixed with a plastic polymer (polypropylene: PP) and extruded from a small die with a diameter of 2 mm kept at 200°C. The threads of NT/PP blend are characterized by small angle X-ray diffractometry and transmission electron microscope to confirm the existence of nanobundle of NTs, which orient along the spinning direction.
Buckytube Cold Field Emitter Array Cathode Experiments
Authors: Fishbine B. H. , Miglionico C. J. , Hackett K. E. , Hendricks K. J. , Wang X. K. , Chang R. P. H. , Shovlin J. D. , Kordesch M. E.
Published: April 2011

We discuss experiments and theory directed towards the development of a buckytube cold field emitter array electron cathode.
Nanotube Carbon Structure Tips - A Source of High Field Emission of Electrons
Authors: Chernozatonskii Leonid A. , Gulyaev Yu.V. , Kosakoyskaja Z.Ja. , Sinitsyn N.I. , Torgashov G.V. , Fedorov E.A. , Zakharchenko Yu.F. , Val'chuk V.P.
Published: April 2011

We present the finding of experiments of considerable field emission from the films consisting of nanotube carbon structureson various substrates (Si, quartz, glass): density of emission current was up to 1-3 A/cm2 while electric field was about 100 V/μ. The “reconstruction” and “inversion” of field emission have also been observed after current breakdown.
Microstructures of Ultrathin Films of Sputtered PbTiO3
Authors: Wasa Kiyotaka , Satoh T. , Tab Ata K. , Adachi H. , Ichikawa Y. , Setsune K.
Published: April 2011

Ultrathin films of perovskite PbTiO3, 10–100nm thick, were epitaxially grown on miscut (001)SrTiO3 substrate by rf-magnetron sputtering at 600°C. The electron microscope and high resolution x-ray diffraction analysis suggested the evidence of epitaxial growth of (001)PbTiO3/(001)SrTiO3 with three dimensional crystal orientation. The stoichiometric film shows extremely smooth surface with the surface roughness less than 3nm. Deposition on a miscut substrate under stoichiometric conditions is essential to make continuous thin films of single crystal perovskite PbTiO3.
A Study of Switching Behavior in Pb(Zr,Ti) O3 Thin Films Using X-Ray Diffraction
Authors: Eatough Michael O. , Dimos Duane , Tuttle Bruce A. , Warren William L. , Ramesh R.
Published: April 2011

Pb(Zr,Ti)O3 (PZT) thin films are being developed for use in optical and electronic memory devices. To study ferroelectric switching behavior, we have produced relatively untextured PZT thin films on Si substrates. We have developed a method for using x-ray diffraction to observe domain switching in situ. Our study involved the use of a micro-diffractometer to monitor the switching behavior in relatively small (0.7mm diameter) electroded areas. Diffraction analyses were done while DC voltages were applied and removed, representing several places in the hysteresis loop. In particular, we were looking for relative intensity changes in the [h 00],[00l] diffraction peaks as a function of position in the hysteresis loop. Our study indicates that the 90° domain switching exhibited by bulk ferroelectrics, is very limited in films on Si when grain sizes are less than about 1 μm.
Energy Levels of Point Defects in Perovskite Oxides
Authors: Robertson J. , Warren W. L.
Published: April 2011

The band structure and energy levels of defects in BaTi3, PbTiO3 and PbZrO3 are calculated and compared. The band gap of Pb(Zr,Ti)O3 varies little because the band edges are formed of Pb 6s and 6p states. The charged Ti and O vacancies are found to be shallow. Transition metal impurities at the B site are found to give rise to numerous charge states because of their high intra-atomic electron repulsion. The levels of Cr4+, Fe4+ and Co4+ lie near midgap. Ti3+ trapped electron centers become deep at higher Zr contents in PZT.
Vacancy Related Defects in Thin Film Pb(ZrTi)O3 Materials
Authors: Krishnan A. , Keeble D.J. , Ramesh R. , Warren W.L. , Tuttle B.A. , Pfeffer R.L. , Nielsen B. , Lynn K.G.
Published: April 2011

Positron annihilation techniques have been applied to characterize vacancy-related defects in ferroelectric thin film structures. Variable energy positron beam measurements were carried out on doped and undoped Pb(Zr,Ti)O3 (PZT) samples subjected to different post-deposition cool down and anneal conditions. The PZT was deposited by sol-gel with either with platinum or RuO2 electrodes, or by laser ablation with La0.5Sr0.5CoO3 electrodes. The RuO2 and La0.5Sr0.5CoO3 electrode samples showed a smaller S-parameter compared to those deposited with Pt electrodes consistent with an improved PZT layer quality. For laser ablated samples cooled in a reducing ambient an increase in S-parameter for both the PZT and La0.5Sr0.5CoO3 layers was observed indicating an increase in neutral or negatively charged open-volume defects.
Spectroscopic Ellipsometry Characterization of La2Ti2O7 Thin Films
Authors: Dickerson Bryan D. , Nagata Masaya , Song Y.J. , Nam H.D. , Desu Seshu B.
Published: April 2011

Optical properties of La2Ti2O7 thin films were investigated by spectroscopie ellipsometry and compared to those of bulk sintered ceramics. Thin films were prepared by pulsed laser deposition (PLD) from bulk targets. To separate the effects of thickness, porosity, and index of refraction on observed Ψ and δspectra in thin films, a Cauchy model for n vs. λ was developed from sintered samples, with known porosity. Assuming the effective bulk index of refraction followed the rule of mixtures, corrected models for La2Ti2O7 without porosity were used to determine thickness and porosity of thin films as a function of fabrication parameters such as laser energy, substrate material and temperature. Ellipsometry models were tested and refined through XRD, EDX, ESCA, and SEM.
Study on Multilayered Electrodes for Ferroelectric Thin Film Capacitors
Authors: Chung Ilsub , Lee J.K. , Lee Wan In , Chung C.W.
Published: April 2011

The ferroelectric properties of PZT on RuO2 electrodes were compared to those on RuOStudy on Multilayered Electrodes for Ferroelectric Thin Film Capacitors. It seems that the microstructure of PZT film plays an important role in the improvement of electrical properties. In particular, the change in the grain size of PZT film is responsible for the enhancement of the electrical properties. It was found in this study that films with smaller grain size had larger coercive fields. The exact dependence of the grain size on the hysteretic property is not fully understood at present. However, it is believed that the grain size affects both domain formation and domain pinning because of the influence of grain boundaries. The change in physical properties of PZT films due to the bottom electrode is understood in terms of interfacial modifications. Here we report on a method to modify physical properties of PZT films utilizing interfacial engineering.