bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of the radio frequency hollow cathode to the microwave antenna discharge for plasma processing
Authors: Bárdoš L. , Baránková H. , Welzel Th. , Dani I. , Peter S. , Richter F.
Published: July 2002

Two nonconventional systems, the radio frequency hollow cathode discharge (RHCD) and the microwave antenna discharge (MWAD), with almost identical geometry of electrodes generating a nitrogen plasma at power level up to 60 W were compared. Both systems were used for deposition of nitride films at similar experimental parameters. The Al–N films were deposited in the RHCD system by reactive physical vapor deposition (PVD) using an Al radio frequency hollow cathode and the CNx films were deposited in the MWAD system by plasma activated chemical vapor deposition (PACVD) from N2+1% (alternatively 0.5% or 0.4%) C2H2 gas mixtures. The vibrational temperatures of nitrogen molecules in both systems were compared as functions of experimental parameters and discussed with respect to the film growth rates in the particular systems. It was found that irrespective of frequency difference of two orders of magnitude the vibrational temperatures of nitrogen molecules were similar, between 3000 and 4600 K, in both systems at similar experimental conditions. However, shapes of dependences of the vibrational temperature on particular parameters were different, due to different plasma generation principles. The nitride film growth rates were found to correlate to the vibrational temperatures of nitrogen molecules, but their dependences on experimental parameters were affected by specific features of the plasma generation in individual systems as well as by different mechanisms of the PVD and the PACVD of films.
Midinfrared surface-emitting PbSe/PbEuTe quantum-dot lasers
Authors: Springholz G. , Schwarzl T. , Heiss W. , Bauer G. , Aigle M. , Pascher H. , Vavra I.
Published: July 2002

Midinfrared laser emission from self-organized PbSe quantum dots in a high-finesse vertical-cavity surface-emitting laser structure is reported. The structure was grown by molecular-beam epitaxy and consists of high reflectivity epitaxial EuTe/PbEuTe Bragg mirrors with a PbSe/Pb1−xEuxTe quantum-dot superlattice as the active region. Narrow laser emission at 4.2–3.9 μm induced by optical pumping is achieved at temperatures up to 90 K. The observation of simultaneous two-mode emission indicates a width of the inhomogeneously broadened PbSe dot gain spectrum of about 18 meV.
Field-induced coordinates for the determination of dynamic vibrational nonlinear optical properties
Authors: Luis Josep M. , Duran Miquel , Kirtman Bernard
Published: July 2002

The most important contribution to nonresonant vibrational nonlinear optical properties arises from nuclear relaxation (NR). In previous work a set of static field-induced vibrational coordinates (FICs) has been developed to simplify calculation of the NR contribution within the infinite optical frequency approximation. Although the number of FICs is small and independent of molecular size, these coordinates form a complete set. However, the infinite optical frequency approximation does not take into account the frequency dispersion, which we evaluate for three conjugated organic molecules that span a range of polarity and valence-bond/charge transfer characteristics. Our results show that dispersion can be significant and that, in such cases, frequency-dependent FICs (FD-FICs) are necessary for an adequate treatment. A complete, though still small, set of FD-FICs is presented for this purpose. Computational tests reveal that a reduced subset, together with an harmonic approximation, can be used to achieve high accuracy outside the infrared (IR) region. That subset is complete for the electro-optic and Pockels and Kerr effects though not for other common properties.
Temperature dependence of the optical energy gap and Urbach’s energy of CuIn5Se8
Authors: Rincón C. , Wasim S. M. , Marı́n G. , Márquez R. , Nieves L. , Pérez G. Sánchez , Medina E.
Published: July 2002

The optical properties of the ordered defect compound CuIn5Se8, which crystallizes in a hexagonal structure, have been studied by the absorption technique. The analysis of the data shows that the band gap energy EG varies from 1.23 to 1.13 eV between 10 and 300 K. It is found that the variation of EG with temperature is due to the contribution of both acoustic and optical phonons with a characteristic phonon energy of about 14 meV. The optical absorption coefficient just below the absorption edge varies exponentially with photon energy indicating the presence of Urbach’s tail. The phonon energy hνp associated with Urbach’s tail, which is found to be 53 meV, is higher than the highest optical phonon mode reported for this compound, which is about 29 meV. The origin of the additional energy is attributed to the contribution of localized modes produced by structural disorder of low energy formation. An empirical relation, also used earlier in the case of 1:1:2 and other ordered defect compounds of the 1:3:5 phases, explains very well the variation of Urbach’s energy with temperature.
Molecular photodesorption from single-walled carbon nanotubes
Authors: Chen Robert J. , Franklin Nathan R. , Kong Jing , Cao Jien , Tombler Thomas W. , Zhang Yuegang , Dai Hongjie
Published: July 2002

Probing the photoelectrical properties of single-walled carbon nanotubes (SWNTs) led to the discovery of photoinduced molecular desorption phenomena in nanotube molecular wires. These phenomena were found to be generic to various molecule–nanotube systems. Photodesorption strongly depends on the wavelength of light, the details of which lead to a fundamental understanding of how light stimulates molecular desorption from nanotubes. The results have important implications to nanotube-based molecular electronics, miniature chemical sensors, and optoelectronic devices.
Towards extending the applicability of density functional theory to weakly bound systems
Authors: Wu X. , Vargas M. C. , Nayak S. , Lotrich V. , Scoles G.
Published: July 2002

While the attempts currently in progress in several groups for the rigorous inclusion of dispersion interactions in density functional theory (DFT) calculations mature and evolve into practical methodology, we contribute to the debate on the applicability of current functionals to the calculation of weak interaction with a systematic investigation of a few, typical, weakly bound systems. We have used both pure DFT and a hybrid approach in which the total interaction energy is partitioned into two parts: (a) the dispersion energy which, in a first approximation is the contribution due to intermonomer correlations and (b) all other interactions. The first component is accurately obtained at all distances of interest by means of a well-known damped multipolar expansion of the dispersion energy while for the second component different approximations will be evaluated. The need to avoid double counting a fraction of the correlation energy when using the hybrid approach and the choice of the appropriate functional are also discussed. We consider four systems of increasing binding strength, namely the Ar2 and Kr2 dimers, the benzene dimer, the water dimer, and a few metal carbonyls. For pure DFT calculations we confirm the conclusion reached by others concerning (a) the strong dependence of the results on the choice of the GGA functional for dispersion-dominated interaction (noble gases and benzene) with the overall tendency to yield underbinding and (b) the relatively accurate, functional-independent, description for that DFT gives of water, which we attribute to the fact that this system is dominated by electrostatic interactions. For the carbonyls we find that DFT yields results which area again strongly dependent on the choice of the functional and show a tendency to give overbinding. Our hybrid method shows instead shortcomings only for the noble gases. The problem in this case is traceable to the well-known difficulties that all current functionals experience at medium–large intermonomer separations. The quality of the hybrid results improves markedly for benzene due to the large value of both dispersion and repulsive interactions at the equilibrium distance for this dimer, which makes the balance between the two, less delicate. Excellent results are also obtained for water (for the same reason as indicated above) and more significantly for the carbonyls where we find that dispersion contributes to the binding more than it could be guessed a priori. We do not claim to have found a general solution to this difficult problem, but we aim at providing a quantitative assessment to where the problems are pointing at directions from which a general solution may, eventually, emerge.
Thermomagnetic detection of recrystallization in FeCoNbBCu nanocrystalline alloys
Authors: Blázquez J. S. , Conde C. F. , Conde A.
Published: July 2002

The recrystallization process in FeCoNbBCu nanocrystalline alloys is evidenced from thermomagnetic results as a significant decrease in magnetization at the second crystallization stage. The lowering in the volume fraction of α-FeCo crystals indicates that some of these crystals contribute to the boride phases formed. Electron microscopy images reveal that the final microstructure consists of large crystals (∼500 nm) of a fcc (FeCo)23B6 phase and small crystals (∼20 nm) of bcc α-FeCo and of some boride phases as such (FeCo)2B.
InAs/GaAs single-electron quantum dot qubit
Authors: Li Shu-Shen , Xia Jian-Bai , Liu Jin-Long , Yang Fu-Hua , Niu Zhi-Chuan , Feng Song-Lin , Zheng Hou-Zhi
Published: July 2002

The time evolution of the quantum mechanical state of an electron is calculated in the framework of the effective-mass envelope function theory for an InAs/GaAs quantum dot. The results indicate that the superposition state electron density oscillates in the quantum dot, with a period on the order of femtoseconds. The interaction energy Eij between two electrons located in different quantum dots is calculated for one electron in the ith pure quantum state and another in the jth pure quantum state. We find that E11〉E12〉E22, and Eij decreases as the distance between the two quantum dots increases. We present a parameter-phase diagram which defines the parameter region for the use of an InAs/GaAs quantum dot as a two-level quantum system in quantum computation. A static electric field is found to efficiently prolong the decoherence time. Our results should be useful for designing the solid-state implementation of quantum computing.
Adsorption and separation of hydrogen isotopes in carbon nanotubes: Multicomponent grand canonical Monte Carlo simulations
Authors: Challa Sivakumar R. , Sholl David S. , Johnson J. Karl
Published: July 2002

Adsorption isotherms of hydrogen isotopes from molecular simulations in carbon nanotubes and interstices are presented. The adsorption of pure isotopes follows Henry’s law up to moderate coverages. A modified path integral grand canonical Monte Carlo (PI-GCMC) technique for mixture adsorption is presented and applied to adsorption of isotope mixtures in carbon nanotubes. Adsorption isotherms of H2–T2 mixtures in nanotubes and interstices are determined at 20 and 77 K. Selectivities for T2 over H2 are calculated over a range of pressures. Selectivity in the nanotubes and interstices increases with pressure until the nanotube is saturated. Comparison of simulation results with predictions based on ideal adsorbed solution theory (IAST) shows good agreement up to moderate loadings. At higher loadings, selectivities determined from multicomponent simulations remain roughly constant, whereas IAST predicts continued increase in selectivities. Isotherms for H2–D2 and the selectivities of D2 over H2 are determined for adsorption in (10,10) nanotubes and in the interstitial channels of closed (10,10) nanotubes.
Kolmogorov two-thirds law by matched asymptotic expansion
Authors: Lundgren Thomas S.
Published: July 2002

The Kolmogorov two-thirds law is derived for large Reynolds number isotropic turbulence by the method of matched asymptotic expansions. Inner and outer variables are derived from the Karman–Howarth equation by using the von Karman self-preservation hypothesis. Matching the resulting large Reynolds number asymptotic expansions yields the Kolmogorov law. The Kolmogorov similarity hypotheses are not assumed; only the Navier–Stokes equation is employed and the assumption that dissipation is finite. This indicates that the Kolmogorov results are a direct consequence of the Navier–Stokes equations.
Electrorheological fluid with an extraordinarily high yield stress
Authors: Zhang Yuling , Lu Kunquan , Rao Guanghui , Tian Yu , Zhang Shaohua , Liang Jingkui
Published: July 2002

Surface modified complex strontium titanate microparticles are synthesized by means of a modified sol–gel technique. A suspension composed of these particles immersed in a silicone oil exhibits excellent electrorheological properties attractive to industry and technology applications: a yield stress as high as 27 kPa in an applied electric field of 3 kV/mm, a low leakage current, wide dynamic ranges in temperature and shear rate, and a long-term stability against sedimentation. In addition to the high dielectric constant of strontium titanate, surfactant and water-free character of the particles may be responsible for the dramatic improvement of the electrorheological properties of the suspension.
Diode and transistor behaviors of three-terminal ballistic junctions
Authors: Xu H. Q.
Published: July 2002

We propose new applications of three-terminal ballistic junctions (TBJs) in nanoelectronics. It is found that for a TBJ with one of its three branch contacts, say the right branch contact, being grounded, the output voltage, Vc, from the central branch as a function of the voltage, Vl, applied to the left branch shows a diode characteristic: Vc approximately follows Vl linearly when Vl is negative, and saturates at a small positive value when Vl becomes positive. It is also found that the saturation level of Vc as well as the threshold value of Vl, beyond which Vc saturates, can be modulated by application of a voltage, Vr, to the right branch contact of the TBJ. Thus, the TBJ can also be used as a transistor. TBJ diodes and TBJ transistors with dimensions on the order of sub 100 nm, or much less, can be made from standard high-quality electronic materials with state-of-the-art nanofabrication technology.
Competing growth kinetics in simultaneously crystallizing and phase-separating polymer blends
Authors: Wang Howard , Shimizu Katsumi , Kim Hongdoo , Hobbie Erik K. , Wang Zhi-Gang , Han Charles C.
Published: July 2002

The kinetic interplay between crystal superstructure growth and late-stage liquid phase coarsening in a polymer blend has been examined. By controlling the relative quench depths for liquid–liquid phase separation and crystallization, the growth kinetics of the characteristic length scales of the simultaneous ordering processes show a crossover from crystallization dominated to phase-separation dominated behavior. Based on a scaling argument for late-stage coarsening during spinodal decomposition, we argue that this kinetic crossover is inevitable in a blend for which the critical temperature of liquid–liquid phase separation is well above the equilibrium melting temperature of the blend.
Recombination mechanisms in p-type HgCdTe: Freezeout and background flux effects
Authors: Schacham S. E. , Finkman E.
Published: July 2002

The recombination mechanisms in HgCdTe are analyzed. Detailed expressions for the radiative lifetime are presented, taking into account recent measurements of the absorption coefficient. In p-type material, carrier freezeout is shown to increase the lifetime exponentially for the radiative and the Auger processes at low temperatures. The effect of background flux is introduced, taking into account its variation with temperature due to the change in energy gap. Lifetime measurements on p-type samples are in good agreement with combined Auger 7 and radiative mechanisms, where the Auger process is more effective for low x values. Highly compensated materials are dominated by the Shockley–Read recombination. For all samples, the intrinsic region is controlled entirely by the Auger process.
Transport and persistent photoconductivity in atomic-planar-doped GaAs-AlAs/GaAs heterostructures
Authors: Prasad S. , Wei H. P. , Tsui D. C. , Schlapp W. , Weimann G.
Published: July 2002

We have observed a striking photoeffect in an atomic-planar-doped GaAs-AlAs/GaAs heterostructure in Shubnikov–de Haas measurements at 4.2 K. After successively illuminating the sample with a GaAs0.6P0.4 red light-emitting diode (LED), both the 2D electron density and mobility decrease initially before they start to increase upon futher illumination. This negative persistent photoconductivity is not observable when the sample is illuminated with GaAs and In0.53Ga0.47As LEDs. On the other hand, when the light dosage is large enough the usual positive persistent photoconductivity is observed with all three LEDs used. The existence of both donorlike traps and acceptorlike traps in our sample is proposed to explain these observations.
CdS microcrystallites-doped thin-film glass waveguides
Authors: Jerominek H. , Pigeon M. , Patela S. , Jakubczyk Z. , Delisle C. , Tremblay R.
Published: July 2002

Thin films of Corning 7059, glass doped with CdS (2%–4% by weight) microcrystallites, with diameters presumably larger than 6 nm, were fabricated using rf sputtering technique. Sharp optical cutoff near 500 nm is visible on the absorption spectra of the films. Both Raman and absorption spectra presumably reveal the presence of size effects. At λ=632.8 nm the refractive index of the films varies from 1.55 to 1.69 depending on rf power. Propagation losses for TE0 guided mode are about 10 dB/cm.
The effects of target compliance on liquid drop impact
Authors: Field J. E. , Dear J. P. , Ogren J. E.
Published: July 2002

When a liquid drop impacts a solid surface, the contact periphery at first expands more quickly than the compression wavefronts in either liquid or solid. The liquid behind the shock envelope is compressed and high pressures of order ρCV result, where ρ is the density of the liquid at ambient pressure, C the shock velocity in the liquid, and V the impact velocity. At a later stage, the shock envelope overtakes the contact periphery and a jetting motion, which releases the high pressures, commences. The magnitude and duration of the high pressures are critical in explaining the damage mechanisms and erosion processes caused by liquid impact. The experiments described in this paper use the two-dimensional gel and photographic techniques developed for visualizing the shocks, recording the onset of jetting, and measuring jet velocities. This particular study is primarily concerned with the effect of target compliance on the early stages of impact. It is shown that the greater the target compliance, the longer the delay before jetting commences. Two critical conditions are shown to be useful in discussing jetting. The first defines when the shock envelope overtakes the contact periphery and liquid can ‘‘spall’’ into the air gap. The second defines when this spalled liquid appears ahead of the contact periphery as an observable jet. Both these conditions are investigated and the implications of the results for erosion damage are discussed.
Modeling and control of diffusion and low-pressure chemical vapor deposition furnaces
Authors: De Waard H. , De Koning W. L.
Published: July 2002

In this paper a study is made of the heat transfer inside cylindrical resistance diffusion and low-pressure chemical vapor deposition furnaces, aimed at developing an improved temperature controller. A model of the thermal behavior is derived which also covers the important class of furnaces equipped with semitransparent quartz process tubes. The model takes into account the thermal behavior of the thermocouples. It is shown that currently used temperature controllers are highly inefficient for very large scale integration applications. Based on the model an alternative temperature controller of the linear-quadratic-Gaussian type is proposed which features direct wafer temperature control. Some simulation results are given.
Dynamics of picosecond laser-induced density, temperature, and flow-reorientation effects in the mesophases of liquid crystals
Authors: Khoo I. C. , Lindquist R. G. , Michael R. R. , Mansfield R. J. , LoPresti P.
Published: July 2002

A detailed theoretical and experimental study of laser-induced density and temperature changes, and flow-reorientation effects in the nematic and smectic phases of liquid crystals is presented. Using picosecond lasers, the initial nanosecond dynamics of the photoinduced density waves, temperature buildup, and relaxations are temporally resolved. The experimentally observed relaxation phenomena and time scales are in good agreement with the theoretical expressions obtained by analytical solutions of the coupled hydrodynamical equations describing these fundamental mechanisms. Our new measurement and theory provide a quantitative account of the relative contribution from the electrostrictive and thermoelastic contributions that had not been presented in previous studies. Our study of the smectic phase has conclusively established the mechanism for the formation of erasable and permanent grating effects under short-laser-pulse excitation as laser-induced electrostrictive and thermoelastic effects.
Experimental studies of high-power microwave reflection, transmission, and absorption from a plasma-covered plane conducting boundary
Authors: Destler W. W. , DeGrange J. E. , Fleischmann H. H. , Rodgers J. , Segalov Z.
Published: July 2002

Experimental studies of the reflection, transmission, and absorption of high-power microwave pulses from a plasma-covered plane conducting boundary are presented. Under optimum conditions, backscattered rf power is attenuated by more than 30 dB over values measured in the absence of the plasma. Measurements of the radial and axial plasma density profiles and the neutral gas pressure near the plane conductor indicate that collisional absorption processes are not the primary source of the observed attenuation in the backscattered microwave signal, and that the plasma density exceeds the critical density over much of the volume nearest the conductor. The effects of a tenfold reduction in the microwave power density on the reflection and absorption characteristics of the system are also reported.
Size dependence of plasmon energy in Si clusters
Authors: Mitome M. , Yamazaki Y. , Takagi H. , Nakagiri T.
Published: July 2002

Plasmons in Si clusters were investigated by electron energy loss spectroscopy attached to high-resolution transmission electron microscopy, by which an individual cluster can be investigated with the electron probe of 2 nm size. It has been found that the plasmon energy increases in proportion to the inverse square of the cluster size, and this is caused by the increase of the energy gap due to the quantum confinement effect.
Variable range hopping in polypyrrole films of a range of conductivities and preparation methods
Authors: Maddison D. S. , Tansley T. L.
Published: July 2002

The electronic transport mechanism in polypyrrole is discussed in terms of Mott variable range hopping (VRH) in samples with a wide range of conductivities and which have been formed using different doping techniques. Samples were synthesized in both aqueous and organic media and samples were either doped during polymerization or dedoped afterwards giving a three order of magnitude range of conductivities at 300 K and a range of sample morphologies. No difference in behavior is observed for materials with different morphologies, suggesting that transport predominantly involves monomer units and occurs independent of structure. The various transport parameters obtained appear reasonable with the exception of the apparent ‘‘hopping attempt frequency,’’ related to the phonon frequency in VRH, some values of which are anomalously high. The density of states at the Fermi level was found to be between 5×1018 and 1×1022 eV−1 cm−3 for a range of samples and the mean hopping distances ranged between 2 and 34 monomer units. The minimum hopping distance of 2 monomer units is consistent with electron delocalization on individual monomer units. An upper limit of conductivity in polypyrrole of no more than 400 S cm−1 is suggested in the limit of the VRH regime in which hopping occurs between adjacent monomer units.
Reaction kinetics on diamond: Measurement of H atom destruction rates
Authors: Harris Stephen J. , Weiner Anita M.
Published: July 2002

There is presently a considerable effort to understand the chemistry of diamond film growth by chemical-vapor deposition. The first measurements of reaction kinetics between diamond and a gas-phase species—H atoms—involved in its formation are described. A remarkably simple method to measure H atom concentrations is developed and the method is used to measure γd, the destruction probability of H atoms on diamond at 20 Torr and 1200 K. It is found that γd=0.12, with an estimated uncertainty of a factor of 2. This value is about half that estimated from gas-phase alkane rate constants and very close to that predicted by molecular-dynamics/Monte Carlo calculations. The agreement to within the estimated uncertainty supports the assumption that gas-phase alkane rate constants are essentially transferable to reactions between a diamond surface and gas-phase species. The H atom measurement technique could be useful for studying growth in other chemical-vapor-deposition systems in which hydrogen is in high concentration.
Modeling of pulsed-laser cleaning of metal optical surfaces at cryogenic temperatures
Authors: Jette A. N. , Benson R. C.
Published: July 2002

Pulsed-laser cleaning of metal optical surfaces at cryogenic temperatures is modeled. Temperature dependencies of the substrate thermal and optical parameters were taken into account since they are quite sensitive to thermal variations at very low temperatures. Calculations of the heating of clean substrates were done to determine damage thresholds as well as computations of the heating of absorbing and transparent films on the substrates to ascertain the film removal process. The theoretical results are compared to available experimental measurements.
Optical properties of tin-doped indium oxide determined by spectroscopic ellipsometry
Authors: Gerfin Tobias , Grätzel Michael
Published: July 2002

Sputtered tin-doped indium oxide (ITO) coatings were investigated by UV-visible spectroscopic ellipsometry at different angles of incidence. The results were fitted using a classical dispersion formula for the optical constants of ITO in a model structure with a compact homogeneous film and a top layer representing surface roughness. Typical values for the high-frequency dielectric constant ε∞ and the plasma frequency ωp were found to be 3.4–4.0 and 0.68–0.96 eV, respectively. The influence of oxidative and reducing annealing at 400 °C was studied. No difference was observed under reducing conditions, but oxygen annealing reduced the influence of the plasma frequency on the studied UV-visible region significantly. To prove the usefulness of the dispersion formula, TiO2 and Nb2O5 films were grown on ITO by metalorganic chemical vapor deposition using Ti(acac)2(i-pro)2 and Nb(OEt)5 as precursors. These bilayers were studied by spectroscopic ellipsometry and could be successfully described applying the previously determined optical constants of ITO and a transparent Cauchy dispersion formula for the deposited oxide.
Cathodoluminescence and photoluminescence in the core region of Bi12GeO20 and Bi12SiO20 crystals
Authors: Cremades A. , Santos M. T. , Remón A. , García J. A. , Diéguez E. , Piqueras J.
Published: July 2002

Cathodoluminescence and photoluminescence of Bi12SiO20 and Bi12GeO20 samples are studied. Both kinds of samples show a dark orange central part or core. Emissions in the blue, green, and red spectral regions are observed. The 640 nm band is the predominant feature in the core emission. The evolution of this band during electron irradiation suggests possible emission mechanisms. In addition, two infrared emissions at about 915 and 1390 nm are detected.
Polarization of thick polyvinylidene fluoride/trifluoroethylene copolymer films
Authors: Chan H. L. W. , Zhao Z. , Kwok K. W. , Choy C. L. , Alquié C. , Boué C. , Lewiner J.
Published: July 2002

Thick (800 μm) polyvinylidene fluoride/trifluoroethylene (P(VDF-TrFE)) copolymer films for transducer applications are poled under applied voltage at elevated temperatures. By using different heat treatments, poling temperatures, and poling time, we are able to prepare a uniformly poled film with a single resonance peak at 1.3 MHz, or a nonuniformly poled film with two resonances (1.3 and 2.6 MHz), or a film with bimorph structure with a single resonance at 2.6 MHz. The nonuniform polarization which arises from charge injection from the cathode is checked by the pressure wave propagation method. The polarization mechanisms in these thick films are expected to be similar to those previously reported for thin films. The results obtained in this work may lead to practical applications because they suggest a means for controlling transducer frequency by poling.
Does the low-temperature Arrhenius plot of the photoluminescence intensity in CdTe point towards an erroneous activation energy?
Authors: Krustok Jüri , Collan Heikki , Hjelt Kari
Published: July 2002

Several experimental photoluminescence (PL) bands of different energies for variously prepared CdTe samples are compared. Temperature variation of the PL intensity is modeled with two nonradiative thermal activation energies, of which ET1 is dominant for about T⩽60 K, and ET2 for the upper temperature range of the measurement. The size of ET1 is invariably of the order of a few meV and, although of unclear origin, its magnitude is usually interpreted as an electronic energy level difference over which the carriers escape by thermal excitation. In CdTe the existence of such a small energy level difference ET1 is not easy to explain. On the contrary, we find clear evidence that, at low temperature, the PL intensity reduction with increasing temperature in fact results from the approximately T−2 temperature dependent capture cross sections of the carriers at the recombination centers, and not from a genuine thermal activation energy ET1.
Phase transition in cadmium sulfide single crystals shocked along the c axis
Authors: Tang Z. P. , Gupta Y. M.
Published: July 2002

Cadmium sulfide crystals were shocked along the crystal c axis to peak stresses ranging between 18 and 75 kbar. Stress-time profiles were measured both at the impact surface and after transmission through 1 to 2-mm-thick samples. Detailed analysis of the present data in combination with published static results makes a persuasive case for the completion of the wurtzite to rocksalt phase change in less than 0.2 μs under shock loading. The main findings are: the transition stress is measured to be 32.5±1 kbar; transformation to the final state is a two step process with the first step being too rapid (less than 10 ns) to be observed in our experiments and the second step occurring in 0.1 to 0.2 μs; the transition occurs directly from the elastic state prior to any plastic deformation. The calculated mean stress for the transition is 22.9 kbar in good agreement with the 23 kbar pressure reported in static high pressure studies; the presence of large shear stress has no effect on the transition pressure. Our results suggest that the onset of the phase transition results in plastic deformation and, subsequently, the phase transition and plasticity are coupled under shock loading.
Magnetic hardening mechanism in nanocrystalline Nd2Fe14B with 0.1 at. % addition of Cr, Cu, or Zr
Authors: Raviprasad K. , Ravishankar N. , Chattopadhyay K. , Umemoto M.
Published: July 2002

The nanocrystalline hard magnetic Nd2Fe14B phase plays a crucial role in the development of exchange spring magnets. We report the hardening mechanism of nanocrystalline Nd2Fe14B, as well as the effect of small amounts of magnetic additions on the hardening behavior. It is shown that the hardening mechanism is of the pinning type in the pure Nd2Fe14B phase. Retention of pinning-type behavior, with a marginal increase in coercivity was observed for small additions of Cr or Cu. Addition of Zr changes the mechanism to nucleation type with a decrease in coercivity. Additions of Zr or Cu are thought to form precipitates at grain boundaries. The precipitates containing Cu were found to be effective in pinning the domains and enhancing the coercivity. The Zr-containing precipitates were found to be ineffective in pinning the domain walls and act as nucleation centers for reverse domains, resulting in nucleation-type hardening. The magnetic behavior of Cr-containing samples above the Neel temperature of Cr remains unaltered. This lack of change in the Cr-containing samples combined with the observed lowering of remanence indicates the presence of Cr in solution with Nd2Fe14B. Thus, we conclude that not only the magnetic nature of the impurity, but also its location is important in controlling the coercivity mechanism.
Hydrogen adsorption states at the Pd/SiO2 interface and simulation of the response of a Pd metal–oxide–semiconductor hydrogen sensor
Authors: Eriksson Mats , Ekedahl Lars-Gunnar
Published: July 2002

The response of a Pd–SiO2–Si hydrogen sensor depends on the reaction kinetics of hydrogen on the Pd surface and on the hydrogen adsorption states at the Pd/SiO2 interface. In this work we show that besides the dominating hydrogen adsorption state located on the oxide side of the interface, a second state, resulting in opposite hydrogen polarization, exists. This state is possibly a reminiscence of the hydrogen adsorption state on a clean Pd surface. Taking both states into account, a simulation of the hydrogen response over more than ten decades in hydrogen pressures gives good agreement with published data.
Role of intergrowths in the properties of naturally layered manganite single crystals (invited)
Authors: Bader S. D. , Osgood R. M. , Miller D. J. , Mitchell J. F. , Jiang J. S.
Published: July 2002

Two-layered Ruddlesden–Popper phase SrO(La1−xSrxMnO3)2, with x=0.3 and 0.4, exhibits colossal magnetoresistance, a magnetic anisotropy which is strongly composition-dependent, very little remanence, and an anomalous low-field magnetization (M) plateau between the Curie temperature (TC) and T*∼300 K. The resistivity peaks near TC for both in-plane and out-of-plane currents. The magnetization plateau is not intrinsic to the crystal, but is attributed to intergrowth defects, consisting of one additional or missing SrO blocking layers, as observed in transmission electron micrographs. The intergrowths exhibit interesting two-dimensional magnetic behavior, for both the x=0.3 and 0.4 compositions. For x=0.4, M scales as (1−T/T*)β, with β=0.25±0.02. For x=0.3, the intergrowths exhibit an easy axis in the a-b plane due to the shape anisotropy, while for T<TC, M lies along the c axis.
The effects of electric field on the electronic structure of a semiconductor quantum dot
Authors: Chang Kai , Xia Jian-Bai
Published: July 2002

The effect of electric field on the electronic structure of a spherical quantum dot is studied in the framework of the effective-mass envelope-function theory. The dependence of the energy of electron states and hole states on the applied electric field and on the quantum dot size is investigated; the mixing of heavy holes and light holes is taken into account. The selection rule for the optical transition between the conduction band and valence band states is obtained. The exciton binding energies are calculated as functions of the quantum dot radius and the strength of the electric field.
Photovoltaic properties of conjugated polymer/methanofullerene composites embedded in a polystyrene matrix
Authors: Brabec C. J. , Padinger F. , Sariciftci N. S. , Hummelen J. C.
Published: July 2002

Bulk donor–acceptor heterojunctions between conjugated polymers and fullerene derivatives have been utilized successfully for photovoltaic devices showing monochromatic energy conversion efficiencies above 1%. The photovoltaic response of these devices is based on the ultrafast, photoinduced electron transfer from the conjugated polymer to the fullerene [N. S. Sariciftci and A. J. Heeger, Handbook of Organic Conductive Molecules and Polymers, (Wiley, New York, 1997), pp. 413–455]. In this work we present efficiency data of solar cells based on a soluble derivative of p-phenylene vinylene (PPV), poly [2-methoxy, 5-(3′,7′-dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV), and a highly soluble methanofullerene, [6,6]-phenyl C61-butyric acid methyl ester (PCBM), embedded into a conventional polymer, polystyrene (PS). By the blending of the optimized donor–acceptor components into the conventional polymer matrix, the percolation threshold for photovoltaic response of the three component systems is found to be determined by percolation of the methanofullerene in the polymer matrix. We present current/voltage data of PS–MDMO-PPV–PCBM devices with various PS concentrations as well as photoinduced absorption studies in the infrared [(PIA) Fourier transform infrared] and light induced electron spin resonance studies on the electron transfer in these composites. At low light intensities, the monochromatic power conversion efficiency ηe and the photon carrier collection efficiency ηc of the PS free device are calculated with 1.5% and 18%, respectively.
Effect of Fe doping on magnetic properties and magnetoresistance in La1.2Sr1.8Mn2O7
Authors: Zhang Jun , Wang Fangwei , Zhang Panlin , Yan Qiwei
Published: July 2002

We report the effect of Fe doping on magnetic properties and magnetoresistance (MR) in the layered perovskite La1.2Sr1.8Mn2O7. Fe doping results in the weakening of ferromagnetic (FM) ordering, and the occurrence of a low-temperature spin-glass state in La1.2Sr1.8Mn2−xFexO7. Resistivity increases sharply with increasing Fe concentration. The metal–insulator transition disappears at a very low doping level (x=0.01), but the low-temperature MR effect is enhanced. These effects result from the replacement of Mn3+ by the doped Fe ions, which interrupts the path of double–exchange interaction and destroys the FM metallic state. Due to the anisotropic interaction in the layered perovskite, the magnetic properties and MR effect are more sensitive to Fe doping than those in ABO3-type perovskites.
Composite thin films of CdSe nanocrystals and a surface passivating/electron transporting block copolymer: Correlations between film microstructure by transmission electron microscopy and electroluminescence
Authors: Mattoussi H. , Radzilowski L. H. , Dabbousi B. O. , Fogg D. E. , Schrock R. R. , Thomas E. L. , Rubner M. F. , Bawendi M. G.
Published: July 2002

We present a microscopic characterization of composite thin films made of CdSe semiconductor nanocrystals (quantum dots) dispersed in a polynorbornene-based block copolymer with surface-passivating and electron transport functionalities. We used two types of nanocrystals, CdSe with pure organic capping groups (“bare”) and CdSe–ZnS core-shell nanocrystals, also capped on the outer surface with organic groups. The composite thin films are incorporated in heterostructure light emitting devices where they serve as both the electron transport layer and the emissive layer. A thin layer of self-assembled poly (phenylene vinylene) (PPV) is used as the hole transport layer. We used transmission electron microscopy (TEM) to examine the film structure in plan view and in cross section. The TEM analysis uncovered distinct and complex differences between the microstructures of thin films containing bare and ZnS overcoated nanocrystals. We found a strong correlation between the composite film microstructure and the corresponding electroluminescence (EL) spectra as well as the device performance. For example, the presence of phase separation at low concentration and nanocrystal migration to the surface in films with ZnS overcoated particles, result in a large PPV contribution to the EL spectra. At higher concentrations, particles form connecting paths between the interfaces, resulting in spectra dominated by nanocrystal emission, but with a higher efficiency for bare particles.
Measurement of porous silicon thermal conductivity by micro-Raman scattering
Authors: Périchon S. , Lysenko V. , Remaki B. , Barbier D. , Champagnon B.
Published: July 2002

We present a noncontact and nondestructive method to measure thermal conductivity in layered materials using micro-Raman scattering. This method was successfully applied to monocrystalline silicon whose thermal conductivity was found to be 63 W/m K at about 550 °C and then applied to porous silicon layers. For a 50 μm thick layer with 50% porosity, we found a thermal conductivity of 1 W/m K confirming the thermal insulating properties of this material.
First-principles calculation of band offsets, optical bowings, and defects in CdS, CdSe, CdTe, and their alloys
Authors: Wei Su-Huai , Zhang S. B. , Zunger Alex
Published: July 2002

Using first principles band structure theory we have calculated (i) the alloy bowing coefficients, (ii) the alloy mixing enthalpies, and (iii) the interfacial valence band offsets for three Cd-based (CdS, CdSe, CdTe) compounds. We have also calculated defect formation energies and defect transition energy levels of Cd vacancy VCd and CuCd substitutional defect in CdS and CdTe, as well as the isovalent defect TeS in CdS. The calculated results are compared with available experimental data.
Residual stress and texture in poly-SiC films grown by low-pressure organometallic chemical-vapor deposition
Authors: Hurtós E. , Rodrı́guez-Viejo J.
Published: July 2002

We present a study of the stress state of polycrystalline 3C–SiC thin films grown on a-SiO2/Si(100) substrates by low-pressure chemical-vapor deposition using an organometallic precursor, tetramethylsilane (TMS), that contains both Si and C atoms. Substrate curvature and x-ray stress measurements indicate a change from compressive to tensile stress with increasing deposition temperature. Film thickness and TMS flow also influence the total stress of the films. The different microstructure of the films with growth temperature and the presence of impurities are at the origin of the observed differences in the stress. While samples grown below 1100 °C have a columnar structure, are highly oriented along [111], and show compressive stress, those grown at 1130 °C are randomly oriented, with an equiaxial grain shape, and are tensile stressed. The thermal stress is tensile and relatively constant over the temperature range investigated. We speculate the observed intrinsic stress is composed of a tensile and a compressive component. Within the grain-boundary relaxation model we calculate the intrinsic stress variations with temperature due to differences in grain size and density of grain boundaries. The compressive component originates from the presence of oxygen impurities within the film. Peak broadening analysis of the x-ray signal reveals the existence of important microstresses due to low adatom mobility during the deposition process.
Relationship between band gap and bond length alternation in organic conjugated polymers
Authors: Brédas J. L.
Published: July 2002

We describe calculations of the evolution of the band gap as a function of geometry in conjugated polymers based on aromatic rings: polyparaphenylene, polypyrrole, polythiophene. Our results demonstrate that the gap decreases as a function of increasing quinoid character of the backbone and is thus not minimal in the case of zero bond length alternation, in contrast to the situation found in polyacetylene-like compounds. We stress the consequences of these results for the understanding of the effects of doping and for the design of new organic polymers with small gaps.
Spin dynamics and thermodynamics in solid-state NMR cross polarization
Authors: Levitt M. H. , Suter D. , Ernst R. R.
Published: July 2002

We investigate spin thermodynamic processes taking place during Hartmann–Hahn cross-polarization experiments in solids, in which spin polarization is transferred between two nuclear spin species by the application of two strong rf fields. As is well known, optimum cross polarization is achieved for a particular ratio of the two rf field strengths called the Hartmann–Hahn condition; we find experimentally that this condition provides the optimum transfer not only on kinetic grounds, as is usually supposed, but also for thermodynamic reasons. By measurement of the evolution of the spin observables in a ferrocene single crystal we demonstrate the existence of a quasi-equilibrium state with nonuniform spin temperature and less than maximum entropy. A modified spin thermodynamic theory is developed whose main features are the important role of the dipolar energy for the quasi-equilibrium state and the existence of constants of the motion other than the total energy. A new cross-polarization experiment is suggested which is shown to provide efficient cross polarization even for mismatched Hartmann–Hahn conditions.
Spherical-complex-optical-potential (SCOP) model for electron–monosilane (SiH4) collisions at 30–400 eV: Total (elastic+absorption), momentum transfer, and differential cross sections
Authors: Jain Ashok
Published: July 2002

We report nonempirical quantum mechanical calculations on the total (elastic+absorption), momentum transfer, and differential cross sections for e-SiH4 collisions at intermediate and high energies (30–400 eV). A parameter-free and energy-dependent spherical-complex-optical potential (SCOP) is evaluated for the e–SiH4 system. The real part of the SCOP consists of three local terms, namely static, exchange, and polarization. The static interaction is generated very accurately from near-Hartree–Fock one-center silane wave functions, while the exchange effects are accounted for in the free-electron-gas-exchange (FEGE) model. The polarization potential is evaluated ab initio in a parameter-free approximation of Jain and Thompson. The imaginary term of the total SCOP represents loss of flux due to inelastic channels via an energy-dependent absorption potential calculated from target electron density and short-range static-exchange force in the quasifree model with Pauli blocking [Staszewska et al.; J. Phys. B 16, L281 (1983)]. Two versions of this absorption potential are employed; one with an undistorted density and the other with a polarized density determined approximately from first order target wave functions. The later version is more successful when the final results are compared with experiment. The total SCOP is treated exactly in a partial-wave analysis using the variable-phase approach to yield complex phase shifts. Our final total cross sections compare very well with the only available measurements of Sueoka and Mori. However, below 50 eV, present total cross sections overestimate the experimental data within 10%. The effect of absorption potential is to reduce the elastic cross sections significantly; this reduction is more dramatic in case of the differential cross sections (DCS); for example, the reduced DCS are exposed to more pronounced structure. Interestingly, the e–SiH4 reduced DCS are very close in shape to the corresponding e–Ar cross sections above 60 eV.
Properties of the square-well fluid of variable width. II. The mean field term
Authors: del Rio Fernando , Lira Leonel
Published: July 2002

The perturbation expansion of the Helmholtz free energy of a square-well fluid is analyzed. A closed-form expression is found for the first-order term in that expansion as a function of the density and of the attractive range of the well. The procedure is based on the interpolation of the extreme cases of long and short ranges, for which exact asymptotic results are known. The proposed interpolation formula is tested against Monte Carlo calculations of the same first-order term for different values of the range and the density, obtaining a root-mean-square deviation of 2.2%. Previous analytic approximations as that of van der Waals are discussed.