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We report a detailed study on the orientation dependent growth characteristics, electronic structure, transport, magnetic, and vibrational excitations in atomically flat interfaces of [ La 0.7 Sr 0.3 MnO 3 / LaNiO 3 ] 10 superlattices (SLs) coherently grown on (001/011/111)- SrTiO 3 substrates by the pulsed laser deposition technique. X-ray reflectometry confirms the periodic superlattice stacks from the Kiessig interference fringes and well-defined even interfaces between the nickelate and manganite layers. A complex local atomic environment across the interfaces was noticed, yet trivalent La, divalent Sr, and mixed valent Ni 2 + / 3 + and Mn 3 + / 4 + electronic states prevail at the core level with enhanced relative intensity ratio of the Mn ions in the superlattices grown on (111) oriented SrTiO 3 substrates as compared to those grown on (001) and (011) oriented SrTiO 3. The temperature (5 ≤ T ≤ 300 K) dependence of electrical resistivity ρ (T) analysis reveals 3D variable range hopping model [ ρ (T) = ρ 0 exp ⁡ (T 0 / T) (1 / 4) ] with large magnitude of hopping energies (≥ 40 meV) for the SL-111 system associated with the high energy gap developed by the accumulation of disorderness in the individual constituents of polar layers. Moreover, all SL systems exhibit reduced ferromagnetic ordering temperatures (67 ≤ T C ≤ 110 K) with a low-temperature anomaly (11.4 ≤ T ∗ ≤ 22 K) and a substantial enhancement in the effective exchange interaction ( J e f f ∼ 3.52 meV) having altered ground state-spin configuration S ∼ 1 / 2 different from S = 3 / 2 of La 0.75 Sr 0.25 MnO 3. Nevertheless, the SL-011 system exhibits large anisotropy field H K ∼ 18 kOe and cubic anisotropy constant K 1 ∼ 9.3 × 10 3 J / m 3 in comparison to the other two orientations. The second order two-phonon interaction driven by the local polaronic distortion causes significant changes in the vibrational excitations of the investigated system. Nonetheless, most of the Raman modes follow the substrate-induced, highly oriented epitaxial growth pattern except for two modes ν 4 (326 cm − 1 ) and ν 8 (728 cm − 1 ), which slightly differ in the case of SL-111 superlattices. [ABSTRACT FROM AUTHOR]

Electroactivity of graphene-family nanomaterials and three-dimensional porous architectures is key for various applications at the grand challenges of "energy-water-sensing nexus." It requires well-controlled morphology, manipulation of surface chemistry, interconnected topologic network, as well as electronic properties. Here, we demonstrate by taking advantage of hierarchical mesoporosity, optimized defects number density, nD (edges-plane and pore sites, oxygenated and nitrogenated functionalities), further invoked by synergistic coupling between one-dimensional single-walled carbon nanotube (SWCNT) as "nano" spacers and polymer linker with two-dimensional reduced graphene oxide derived three-dimensional scaffolds (known as aerogels) under hydrothermal conditions, and improved electrochemical (re)activity by enhancing the heterogeneous electron transfer rate (kET). We determined the correlation among nD, in-plane sp2C cluster, La and interdefect distance, LD (all via Raman spectroscopy), and kET (via scanning electrochemical microscopy) to establish "structure–property–functionality–electroactivity" relationships. The prominent Raman bands were also analyzed to determine the sp2-bonded C cluster size (La) for graphene- and nanotube-rich phases. The interplay of (1) rich surface redox chemistry due to carbonyl—C=O, carboxyl—COOH, pryridinic—N and pyrrolic—N functional groups, and geometric defects; (2) protruded edge plane and nanopores sites; (3) topological network; and (4) finite density of states with increased vacancy sites is emphasized and signifies the inherently activated electronic states in functionalized nanoporous composite carbon aerogels, for improved physicochemical processes (following graphene < N-graphene < graphene-SWCNT < N-graphene-SWCNT aerogels) relevant for electrocatalysis, thermo-electrochemical energy harvesting, desalination, and biosensing. [ABSTRACT FROM AUTHOR]

Investigations on Gd-substituted lead magnesium niobate (Pb1-xGdxMg(1+x)/3Nb(2-x)/3O3; varying x = 0.01–0.1) ceramics have revealed critical slowing down of the polar nano regions (PNRs) ensemble into a “super-dipolar glass state” for higher Gd-substitution x ≥ 0.05. Low temperature electric field induced polarization switching study (P-E) has revealed a sharp decrease in the remanent polarization up to x = 0.03, which strengthen the critical slowing down of polar nano-domains dynamics, suggesting a reduction in the correlation between or within polar nano regions (PNRs) leading to a reduction in its size. Bright field imaging by using transmission electron microscope has also confirmed the reduction of the size of polar nano regions with increasing “x.” Selected area electron diffraction pattern along ⟨110⟩ unit axis has revealed enhancement in intensity of the superlattice reflections spot at ½ ½ ½ along ⟨111⟩ unit axis with increasing “x,” which is associated with the enhancement of chemical ordered regions and correlate well to enhancement in the degree of diffuseness parameters “δA” determined from fitting of the temperature dependent dielectric constant ε(T) plot above the dielectric maximum peak (εmax). The enhanced “δA” for x ≥ 0.05 is due to additional disorder created by the Gd-ions substitution at the Mg-site, which is consistent with the phase and microstructural analysis. Fitting of frequency dependent Tm (temperature of εmax) to the power law of critical dynamic has revealed realistic pre-factor fitting parameters for x ≥ 0.05 suggesting critical slowing down of the polar nano-domains dynamics ensemble resulting in super-dipolar glass state. [ABSTRACT FROM AUTHOR]

It has been possible to use spectroscopic ellipsometry from the near infrared to near ultraviolet range to study the layered structure of polycrystalline diamond thin films produced by electron cyclotron resonance-assisted chemical vapor deposition. It is suggested that both high surface mobilities and weak-bond etching can give rise to relatively smooth polycrystalline diamond surfaces.

Research describing electron field emission measurements of nanocrystalline carbon films is presented. In particular a comparison of the properties of films grown under positive, negative and unbiased conditions is examined.

Research describing the microstructure of diamond thin films is presented. The study investigates differences between microwave chemical vapour deposition techniques and methods using electron cyclotron resonance.

Phase transitional, dielectric, electro-optical, polarizing optical microscopic, photoluminescence (PL), and Fourier transformed infrared (FTIR) spectroscopic measurements have been carried out on ZnS:Mn quantum dots (QDs) dispersed ferroelectric liquid crystal (FLC). A new dielectric relaxation mode has been envisaged in FLC material due to the presence of 0.25 wt.% ZnS:Mn (40 mol. %) QDs. The characteristics of the new mode have been compared with those of the soft mode. A significant fastening of the electro-optical response (~75%) has been observed in the case of 0.25 wt.% ZnS:Mn (20 mol. %) QDs doped FLC material. The induction of the new relaxation mode is attributed to the flexoelectric tilt fluctuations. The induced flexoelectric polarization in the FLC medium at the vicinity of QDs might be responsible for the enhanced spontaneous polarization in the FLC/QDs mixtures. Quenching in PL for the FLC/QDs mixtures has been observed, which strongly depends on Mn content in QDs. The change in FTIR spectra for the FLC/QDs composite clearly indicates the change in molecular dynamics of the pure FLC after the dispersion of QDs. The presented results will certainly pave a way to utilize QDs for obtaining faster response of the FLC material and potential material to tune the relaxation processes. [ABSTRACT FROM AUTHOR]

Small angle neutron scattering and gamma-ray diffractometry help analyze the defects in Czochralski-grown silicon single crystals after annealing at 750 degrees Celsius due to precipitation of oxygen. The small SiO2 particles coalesce to form large clusters, in accordance with the Huang and Stokes- Wilson theory. The cluster are at a distance of about 40,000 Angstroms from each other, while the centre of one SiO2 precipitate in the cluster is about 80 Angstroms from the other.

The effect of cadmium selenide quantum dots (CdSe QDs) on the dielectric relaxation and material constants of a ferroelectric liquid crystal (FLC) has been investigated. Along with the characteristic Goldstone mode, a new relaxation mode has been induced in the FLC material due to the presence of CdSe QDs. This new relaxation mode is strongly dependent on the concentration of CdSe QDs but is found to be independent of the external bias voltage and temperature. The material constants have also been modified remarkably due to the presence of CdSe QDs. The appearance of this new relaxation phenomenon has been attributed to the concentration dependent interaction between CdSe QDs and FLC molecules. [ABSTRACT FROM AUTHOR]

The properties of dc magnetron sputter-deposited 45% Ni-55% Fe is presented. The magnetic properties were found to correlate with the crystalline texture, in general, better soft properties arise with less (111) texture.

The optimization of long-throw sputter-deposited hard magnetic CoCrPt/Cr films was undertaken. Increasing the T/S spacing to increase the collimation in the film deposition is detrimental to the magnetic properties, in particular the coercivity.

Effect of fast neutron irradiation induced defects on the anti-ferromagnetic (AFM) to ferromagnetic (FM) transition in Ce(Fe0.96Ru0.04)2 is studied by magnetic measurements. We observe that at a given temperature, AFM-FM transition occurs at a lower field in irradiated specimens compared to that in pristine samples. While irradiation does not seem to affect the hysteresis of the transition, zero field susceptibility is enhanced by more than an order of magnitude. Analysis of reversible magnetization vs field data at low fields indicates superparamagnetic behavior of nano-sized FM domains, which are conjectured to nucleate on the crystal defects produced by the fast neutron irradiation. The number density and size of the nano-sized domains in the irradiated and pristine samples are estimated from the magnetization data. [ABSTRACT FROM AUTHOR]

Magnetic behavior of CoFeB at various thicknesses ranging from 2 nm to 8 nm capped with different materials, such as MgO, Ta, Ru, and V have been studied. The films were sputter-deposited and subsequently characterized by magnetometry and broadband ferromagnetic resonance (FMR). There are magnetically dead layers at the interface observed with Ru and Ta capping layers, while MgO and V have almost no effect on the magnetization of the CoFeB. As the ferromagnetic layer is made thinner, the effective magnetization decreases, indicating an interfacial perpendicular anisotropy. Particularly in the case of MgO, V/Ru, and V/Ta capping layers, interfacial perpendicular anisotropy is induced in CoFeB, and the Gilbert damping parameter is also reduced. The origin of this perpendicular magnetic anisotropy (PMA) is understood to be caused by the interface anisotropy between the free layer and the capping layer. The effect of post-deposition annealing and CoFeB thickness on the anisotropy and damping of V/Ta capped samples are reported. Doping CoFeB with vanadium (V) greatly reduced the 4πMs and 4πMeff values, resulting in an effective increase in the PMA. [ABSTRACT FROM AUTHOR]

Elucidation of microscopic properties of synthetic diamond films, such as formation and evolution of bulk and surface defects, chemistry of dopants, is necessary for a reliable quality control and reproducibility in applications. Surface photovoltage (SPV) spectroscopy and photoluminescence (PL) spectroscopy were employed to study diamond thin films grown on silicon by microwave plasma-assisted chemical vapor deposition and hot-filament chemical vapor deposition with different levels of boron doping in conjunction with gamma irradiation. SPV experiments showed that while the increase of boron concentration leads to a semiconductor-metal transition, subsequent gamma irradiation reverts quasi-metallic samples back to a semiconducting state by compensating electrical activity of boron possibly via hydrogen. One of the most pronounced common transitions observed at ∼3.1-3.2 eV in the SPV spectra was also present in all of the PL spectra. It is likely that this is a signature of the sp2-hybridized carbon clusters in or in the vicinity of grain boundaries. [ABSTRACT FROM AUTHOR]

Effect of ambient gases present in air on photoluminescence (PL) and electrical conductivity of films prepared using ZnO nanoparticles (NPs) have been investigated. It is observed that NPs of size below 20 nm kept inside a chamber exhibit complete reduction in their visible PL when oxygen partial pressure of the surrounding gases is decreased by evacuation. However, the visible PL from ZnO NPs is insensitive to other major gases present in the ambient air. The rate of change of PL intensity with pressure is inversely proportional to the ambient air pressure and increases when particle size decreases due to the enhanced surface to volume ratio. On the other hand, an assembly of ZnO NPs behaves as a complete insulator in the presence of dry air and its major components like N2, O2, and CO2. Electrical conduction having resistivity ∼102-103 Ωm is observed in the presence of humid air. The depletion layer formed at the NP surface after acquiring donor electrons of ZnO by the adsorbed oxygen has been found to control the visible PL and increases the contact potential barrier between the NPs which in turn enhances the resistance of the film. [ABSTRACT FROM AUTHOR]

We report the structure and physical properties of novel hybrids of multiwalled carbon nanotubes (MWCNTs) and ultradispersed diamond (UDD) forming nanocomposite ensemble that were subjected to 50, 100, and 103 kGy gamma ray doses and characterized using various analytical tools to investigate hierarchical defects evolution. This work is prompted by recent work on single-walled CNTs and UDD ensemble [Gupta et al., J. Appl. Phys. 107, 104308 (2010)] where radiation-induced microscopic defects seem to be stabilized by UDD. The present experiments show similar effects where these hybrids display only a minimal structural modification under the maximum dose. Quantitative analyses of multiwavelength Raman spectra revealed lattice defects induced by irradiation assessed through the variation in prominent D, G, and 2D bands. A minimal change in the position of D, G, and 2D bands and a marginal increase in intensity of the defect-induced double resonant Raman scattered D and 2D bands are some of the implications suggesting the radiation coupling. The in-plane correlation length (La) was also determined following Tunistra-Koenig relation from the ratio of D to G band (ID/IG) besides microscopic stress. However, we also suggest the following taking into account of intrinsic defects of the constituents: (a) charge transfer arising at the interface due to the difference in electronegativity of MWCNT C sp2 and UDD core (C sp3) leading to phonon and electron energy renormalization; (b) misorientation of C sp2 at the interface of MWCNT and UDD shell (C sp2) resulting in structural disorder; (c) softening or violation of the q∼0 selection rule leading to D band broadening and a minimal change in G band intensity; and (d) normalized intensity of D and G bands with 2D band help to distinguish defect-induced double resonance phenomena. The MWCNT when combined with nanodiamond showed a slight decrease in their conductance further affected by irradiation pointing at relatively good interfacial contact. Furthermore, owing to high thermal and electrical conductivity properties, they can facilitate potentially efficient heat-transfer applications and some results deduced using Nielsen's model is provided. [ABSTRACT FROM AUTHOR]

We report the structure and physical properties of novel nanocarbon hybrids of single-walled carbon nanotubes (SWCNT) and ultradispersed diamond (UDD) forming truly tetragonal-trigonal nanocomposite ensemble with and without gamma irradiation. They were subjected to 50, 100, and 103 kGy doses and were characterized using analytical tools including electron microscopy, x-ray diffraction, resonance Raman spectroscopy (RS), and electrical measurements. Experiments showed that irradiation generates microscopic defects (the most likely vacancies) in a hierarchical manner much below amorphization threshold (≥103 kGy) and that nanocomposites tend to be radiation resilient, elucidated through the intensity, bandwidth, and position variation in prominent RS signatures. In the interpretation of findings the possibilities for these complex system are: (1) defect-mediated double-resonance mechanism may not explain intensity variation; (2) softening or violation of the q=0 selection rule; (3) difference in electronegativity of sp2 C (SWCNT) and sp3 C (UDD) can result in charge transfer and bond misalignment at the interface; and (4) the nanotubes are stabilized by nanodiamond particles. Furthermore, an attempt was made to identify the nature of defects (charged versus residual) through in-plane correlation length or sp2 C cluster size (La). The decreasing trend of La for both SWCNT and nanocompo sites with gamma irradiation implies charging defects described in terms of dangling bonds in contrast to passivating residual or neutral defects. Moreover, the electrical properties were relatively more labile to irradiation than structural and vibrational properties. [ABSTRACT FROM AUTHOR]

A tunable spatio-temporal Talbot imaging phenomenon is presented. This phenomenon is based on the radiation properties of an array of beam-steered metamaterial composite right-/left-handed leaky-wave antennas, which is excited by a modulated pulse. The scanning law property of these antennas is exploited to achieve off-axis radiation, which leads to a tunable Talbot distance, as a function of the input pulse modulation frequency. The proposed Talbot phenomenon is analyzed theoretically, taking into account the aberrations produced by higher-order terms present in the free-space transfer function. Numerical simulations confirm the self-imaging and pulse multiplication effects and their tunability capabilities as a function of frequency. Finally, the experimental results are included to confirm the phenomenon predicted. [ABSTRACT FROM AUTHOR]

Free vibrations of zigzag single-wall carbon nanotubes (SWCNTs) of aspect ratio (length/diameter) ∼6 and with ends traction-free have been studied using molecular mechanics (MM) simulations with the MM3 potential. It is found that the frequencies of inextensional (the Love and the Rayleigh) modes of an (n, 0) SWCNT saturate at the circumferential wave number of either (n-1)/2 or n/2 where n is odd or even. This is explained in terms of its molecular structure. Since the frequencies of the inextensional modes of vibration of a thin cylinder made of an isotropic linear elastic material do not saturate with an increase in the circumferential wave number, a continuum structure cannot represent all modes of vibration of a zigzag SWCNT. This result is independent of the value assigned to the wall thickness of the SWCNT. We have also found values of material and geometric parameters of a shell and a hollow cylinder by equating their frequencies of the inextensional, the radial breathing, the axial and the torsional modes of vibrations to the corresponding ones of a zigzag SWCNT, and by taking their mean diameter and length equal to those of the SWCNT. The frequencies of the extensional modes of oscillations of the two continuum structures for various axial half wave numbers and circumferential wave numbers are found to match well with those of the SWCNT obtained from the MM simulations. However, the frequencies of the inextensional modes of the continuum structures deviate noticeably from those of the SWCNT, and this deviation increases with an increase in the circumferential wave number. [ABSTRACT FROM AUTHOR]

Herein we report on a systematic investigation of the thermal expansion of select Mn+1AXn phases. The bulk dilatometric thermal expansion coefficient αdil was measured in the 25–1200 °C temperature range and the thermal expansion of more than 15 of these phases was studied by x-ray diffraction in the 25–800 °C temperature range. The coefficient of thermal expansion for the a axis αa ranged between (2.9±0.1)×10-6 °C-1 (Nb2AsC) and (12.9±0.1)×10-6 °C-1 (Cr2GeC) while the coefficient for the c axis (αc) ranged between (6.4±0.2)×10-6 °C-1 (Ta2AlC) and (17.6±0.2)×10-6 °C-1 (Cr2GeC). Weak anisotropy in the thermal expansion was seen in most phases, with the largest value of αc/αa belonging to Nb2AsC. The Grüneisen parameters along the a and c directions were calculated from ab initio values for the elastic compliances and were relatively isotropic. A good correlation was found between the thermal expansion anisotropy and the elastic constant c13 and we conclude that the anisotropy in thermal expansion is related to the bonding between the M-A elements. [ABSTRACT FROM AUTHOR]

A spatial-temporal Talbot phenomenon, based on metamaterial composite right/left-handed (CRLH) leaky-wave antennas (LWAs), is presented. This phenomenon, reported in the microwave domain, is based on the combination of the conventional spatial monochromatic Talbot effect and the transient (polychromatic) character of the pulse radiation phenomenon in the LWA structure. When the elements of a periodic CRLH LWA array are fed simultaneously by an input pulse, the spatial beams corresponding to different temporal frequencies constructively interfere in space so as to form a self-imaged pattern constituted by narrow Talbot zones. This Talbot effect is spatial-temporal since the Talbot zones are localized both in space and time. The phenomenon is analyzed theoretically and validated numerically for the case of narrow-band pulses. [ABSTRACT FROM AUTHOR]

We report the optical property measurements on boron-doped diamond (BDD) films which were synthesized by microwave plasma-assisted chemical vapor deposition technique on Si (100) using methane in high hydrogen dilution and trimethylboron as precursors with varying boron concentration such that [B]/[C]gas=100, 500, 1000, 2000, 4000, and 6467 ppm. These BDD films were investigated using a rotating analyzer variable angle spectroscopic ellipsometry (SE) from the near IR to UV range (830–193 nm). By applying the conventional Bruggeman effective medium approximation and linear regression analyses to the raw SE data that is, [ψ(λi),Δ(λi)] and pseudodielectric function (<[variant_greek_epsilon]r(λi)>,<[variant_greek_epsilon]i(λi)>), we determined the most appropriate model fit. The SE modeling was performed through the normal and point-by-point fit methods combined with the coupled and uncoupled bulk and surface layer approaches providing the details about the thin films’ microstructure in terms of the (a) multilayer (component and surface) structure and component layer thickness of the films, (b)volume fraction of constituents [fsp3 C, fsp2 C and void (fv) in the component layer], (c) inhomogeneity of the structure along the growth axis and its variation with boron concentration, and (iv) surface roughness layer thickness (ds) with dimensions less than the optical wavelength that is not otherwise available. A simplified three-layer structural model consisting of an interfacial layer, an intermediate (or bulk) layer, and a top surface roughness layer has been proposed, which simulates the ellipsometry data reasonably well with coupled point-by-point method. An estimator, i.e., mean squared error (χ2), is used to assess the accuracy of the model fit. The... [ABSTRACT FROM AUTHOR]

Electrical transport has been investigated in amorphous and polycrystalline (α-phase) iron phthalocyanine (FePc) thin films grown by molecular beam epitaxy. Measurements carried out in the temperature range of 150–300 K showed hysteric current-voltage (I-V) characteristics at temperatures above 200 K. The I-V characteristics measured during the increasing voltage scan showed a transition from the Ohmic conduction to the trap controlled space charge limited conduction followed by a trap free conduction. During the decreasing voltage scan, trap free conduction was observed in full voltage range. The I-V hysteresis is attributed to the filling of deep surface traps created by chemisorbed oxygen. Amorphous films showed higher hysteresis as well as chemisorbed oxygen content. [ABSTRACT FROM AUTHOR]

We report the temperature and current dependence of electroresistance (ER) in the polycrystalline (PC) La0.6Pb0.4MnO3 (LPMO) thin films with grain size 55 nm. ER is observed only below metal-insulator transition temperature (TIM). In the temperature range TIM, ER shows a sharp increase at low currents (<50 μA) followed by saturation. The experimental results can be understood in terms of modification of grain boundaries characteristics by spin polarized carriers. ER versus current behavior remains unaffected by the magnetic field, indicating that the mechanism governing ER and magnetoresistance are effectively decoupled. The sharp increase in ER at low currents suggests that PC LPMO films may have potential for application in low power devices operating at low temperatures. [ABSTRACT FROM AUTHOR]

In situ Raman spectroscopy technique was employed to investigate the ion transport process and to determine the concomitant electrochemical tuning of Fermi level in single-wall carbon nanotube. The variation of structural bonding in single-wall carbon nanotube bundle dipped in aqueous alkaline earth halide electrolyte such as CaCl2 with electrochemical biasing was monitored. It is because Raman can detect changes in C–C bond length through radial breathing mode at ∼184 cm-1 that varies inversely with the nanotube diameter and the G band at ∼1590 cm-1 that varies with the axial bond length. Consistent reversible and substantial variations in Raman intensity of both the modes induced by electrode potential point at the fine and continuous tuning (alternatively, emptying/depleting or filling) of the specific bonding and antibonding states. Qualitatively, the results were explained in terms of changes in the energy gaps between the one-dimensional van Hove singularities present in the electron density of states arising possibly due to the alterations in the overlap integral of π bonds between the p orbitals of the adjacent carbon atoms. We estimated the extent of variation of the absolute potential of the Fermi level and overlap integral (γ0) between the nearest-neighbor carbon atoms from modeling the electrochemical potential dependence of Raman intensity. Observations also suggest that the work function of the tube is larger for the metallic nanotubes in contrast to the simultaneously present semiconducting nanotubes. [ABSTRACT FROM AUTHOR]

Plastically deformed (hand-filed) Cu–Sn–5Zn ternary alloys with Sn concentrations 1, 2.5, and 5 wt % are investigated. Microstructural parameters are studied in terms of x-ray diffraction profile fitting analysis. It is observed by Dey et al. [Acta. Mater. 53, 4635 (2005)] that the change in stacking fault probability (α) with Sn concentration for ternary Cu–Sn–5Zn alloys is similar to Cu-based binary alloy (Cu–Sn) system but behaves in a different manner from Cu–1Sn–Zn ternary alloy systems. The crystallite size distribution is broader for alloy with 1 wt % Sn and becomes narrower with increasing Sn concentration. Value of dislocation density (ρ) is of the order of 1015 m-2 and shows a compositional dependence. Type of dislocation is found to be predominantly screw; <100>-type dipoles may also be present in the cold-worked alloys. The dislocation arrangement is found to be more correlated in case of 1 wt % Sn compared to other alloys of higher Sn concentration. The stacking fault energy (γ) is obtained from modified Reed-Schramm equation and is of the order of 20 mJ m-2 with no significant compositional dependence. [ABSTRACT FROM AUTHOR]

In this paper, we report on the heat capacities cp of bulk polycrystalline samples of Ti2AlC, V2AlC, and Cr2AlC in the 3–260 K temperature range. Given the structural and chemical similarities of these compounds it is not surprising that the cp’s and their temperature dependencies were quite similar. Nevertheless, at all temperatures the heat capacity of Cr2AlC was higher than the other two. The density of states at the Fermi level were 3.9, 7.5, and 14.6 (eV unit cell)-1 for Ti2AlC, V2AlC, and Cr2AlC, respectively. The results obtained are analyzed using the Debye and Einstein model approximations for cp. Good description of cp is obtained if one assumes that nine phonon modes vibrate according to the Debye model approximation whereas the remaining 3 of 12 modes expected for M2AlC formula unit fulfill an Einstein-like phonon vibration pattern. Debye temperatures θD describing acoustic phonon and Einstein temperature θE describing optical phonon contributions have been estimated for the studied compounds. The Debye temperatures are reasonably high and fall in the range of 600–700 K. A linear dependence was found between the number of d electrons along the row Ti, V, and Cr and the density of states at the Fermi level. [ABSTRACT FROM AUTHOR]

The transition of the internal structure in microwave chemical-vapor-deposited carbon nanotubes is investigated using scanning electron microscopy and high-resolution transmission electron microscopy. By controlling the thickness of the iron catalyst layer, a sequence of carbon nanotube films was obtained with diameters ranging from a few nanometers to over 100 nm. Experiments have established that by continuous reduction of the Fe layer thickness to <1 nm, single- and double-wall carbon nanotube films can be produced, whereas for an Fe film thickness >1 nm, multiwall carbon nanotube films can be synthesized. It was also found that for an Fe thickness >=5 nm, interlayers (i.e., bamboolike or periodically compartmentalized nanotubes) were formed, while for an iron thickness <2 nm the tubes were primarily hollow. For an intermediate Fe thickness the internal structure of the carbon nanotubes was a mixture of hollow and bamboolike. A growth model which considers bulk and surface diffusions of carbon into and/or onto the Fe catalyst surface is proposed to describe this transition and the internal periodic structure. [ABSTRACT FROM AUTHOR]

Electron field-emission properties of as-grown, etched, and regrown carbon nanotube thin films were investigated. The aligned carbon nanotube films were deposited by the microwave plasma-assisted chemical vapor deposition technique. The surface of the as-grown film contained a carbon nanotube mat of amorphous carbon and entangled nanotubes with some tubes protruding from the surface. Hydrogen plasma etching resulted in the removal of the surface layer, and regrowth on the etched surface displayed the formation of a new carbon nanotube mat. The emission site density and the current-voltage dependence of the field emission from all of the samples were analyzed. The results showed that the as-grown sample had a few strong emission spots and a relatively high emission current density (∼20 μA/cm2 at 1 V/μm), while the regrown sample exhibited a significantly increased emission site density. [ABSTRACT FROM AUTHOR]

Thin films of nanocrystalline diamond were grown by filament-assisted chemical-vapor deposition using methane as carbon precursor with high hydrogen dilution and hydrogen sulfide concentration ranging from 0 to 500 ppm in the gas phase. The surface topography and electronic structure of these films (n-C:S) were investigated using ultrahigh-vacuum scanning tunneling microscopy and scanning tunneling spectroscopy (STS), respectively. Topographic image analyses depict that the root-mean-square roughness of the film surface and average grain size decreases with increasing sulfur incorporation either in gas phase or solid films. High-resolution scanning tunneling microscopy images reveal the localized regions of high conductivity (white) surrounded by less conductive regions (black) pointing at the existence of inhomogeneous mixture of sp2- and sp3-bonded carbon in aggregate or clustered and dispersed state. The surface density of states was determined using scanning tunneling spectroscopy where normalized differential conductivity, i.e., (dI/dV)/(I/V) mimics local density of states (DOS). These methods were employed to understand the role of sulfur in the modification of both the surface microstructure and electronic structure near the Fermi level. The band edges were derived by taking tangents to the differential conductivity (dI/dV) within a certain potential window of ±2 eV of the Fermi level. The resulting band gap is found to be similar to that measured optically (Tauc gap). The Fermi level for undoped nanocrystalline carbon (n-C) was found just below the midgap indicating that n-C is a weakly p-type semiconductor. The STS DOS shows oscillatory behavior or peaks which we ascribe to states of the surface layer having relatively more graphitic or sp2-bonded carbon bonds. With higher sulfur addition, the Fermi level is found to move above the midgap. These results seem to agree quite well with our early work on electrical conductivity exhibiting n-type doping taking place and declining band gap at higher sulfur contents measured with spectroscopic ellipsometry technique on the same samples. [ABSTRACT FROM AUTHOR]

An open question to the community about the general consensus on the field-emission mechanism in carbon-based materials led to this study. By applying the Fowler–Nordheim (FN) model for carbon-based films, despite the fact that the microstructure and the resulting physical properties of the films can be tuned by scanning various process parameters, providing, in turn, from almost insulating (less defective) to semiconducting (highly defective) films and even a mixture of the two, the material can be categorized as electrically heterogeneous nanostructured carbon. The electrical heterogeneity arises from the different carbon hybridizations (sp2- versus sp3-bonded carbon). In an attempt to tackle these issues, we have performed a comprehensive analysis of I–V data obtained from filament-assisted chemical-vapor-deposition-grown sulfur-incorporated nanocomposite carbon thin films with different microstructures. Studies of the augmentation of the field-emission properties in this material indicated various roles of sulfur in modifying the film properties [Gupta et al., Appl. Phys. Lett. 80, 3446 (2002)]. The I–V data were fitted to various mathematical forms: I=AV2 exp(-B/V) [FN model], I=C exp(aV1/2/kT) [Schottky model], and I=Vn (n>1, for high fields) [space-charge-limited current (SCLC) model]. The goodness of fit along with the theoretical justification(s) on the electron field-emission results were taken into consideration to provide favorable indications for accepting or discarding any particular model. These findings suggest that there is an apparent crossover from SCLC to FN behavior as a function of film microstructure occurring due to the impurity incorporation as the microstructure transits smoothly from microcrystalline to nanocrystalline carbon. Other evidence in support of the aforementioned suggestion is based on the concept of percolation occurring in this nanocomposite carbon (a mix of conducting–insulating/semiconducting) material, whereby the electrons are allowed to tunnel from one conductive cluster to another separated by an insulating matrix, which is demonstrated through electrical conductivity measurements. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Charge transfer dynamics on the surface of single-wall carbon nanotube sheets is investigated using in situ Raman spectroscopy in order to understand the actuation mechanism of an electrochemical actuator and to determine associated parameters. We built an actuator from single-wall carbon nanotube mat and studied its actuation in several alkali metal (Li, Na, and K) and alkaline earth (Ca) halide and sulfate solutions in order to clarify the role of counterion as mobile ions in the film. The variation of bonding with applied potential was monitored using in situ Raman spectroscopy. This is because Raman can detect changes in C–C bond length: the radial breathing mode at ∼190 cm-1 varies inversely with the nanotube diameter, and the G band at ∼1590 cm-1 varies with the axial bond length. In addition, the intensities of both the modes vary with the emptying/depleting or filling of the bonding and antibonding states due to electrochemical charge injection. We discussed the variation of peak height and wave numbers of these modes providing valuable information concerning electrochemical charge injection on the carbon nanotube mat surface. We found in-plane microscopic compressive strain (∼-0.25%) and the equivalent charge transfer per carbon atom (f[sub c]∼-0.005) as an upper bound for the actuators studied hereby. It is demonstrated that though the present analysis does comply with the proposition for the actuation principle made earlier, the quantitative estimates are significantly lower if compared with those of reported values. Furthermore, the extent of variation, i.e., coupled electro-chemo-mechanical response of single-wall carbon nanotubes (SWNT) mat depended upon the type of counterion used (Group I versus Group II). The cyclic voltammetry and ac electrochemical impedance spectroscopy results were described briefly, which help to demonstrate well-developed capacitive behavior of SWNT mat and to estimate the specific capacitances as well. Summarizing, the impact of these findings on the suitability of such material for use in electrochemical devices such as actuators is emphasized. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Sulfur incorporated nanocrystalline carbon (n-C:S) thin films were grown on molybdenum substrates by a hot-filament chemical vapor deposition technique using gas mixtures of methane, hydrogen, and a range of hydrogen sulfide (H[SUB2]S) concentrations (100-500 ppm with an interval of 100 ppm) at a fixed substrate temperature of 900 °C. They were optically characterized using Raman spectroscopy (RS) and ex situ spectroscopic phase modulated ellipsometry from near-infrared to near UV (1.5-5.0 eV) obtaining their vibrational frequencies and pseudodielectric function, respectively, as a function of [H[SUB2]S]. The ellipsometry data [〈∈[SUBr](E)〉, 〈(∈[SUBi](E)〉] were modeled using Bruggeman effective-medium theory and dispersion relations for the amorphous semiconductors: Forouhi and Bloomer (FB) parameterization model. A simplified two-layer model consisting of a top layer comprising an aggregate mixture of sp[SUP3]C+sp[SUP2]C+void and a bulk layer (L[SUB2]), defined as a dense amorphized FB-modeled material, was found to simulate the data reasonably well. Through these simulations, it was possible to estimate the dielectric function of our n-C:S material, along with the optical band gap (E[SUBg]), film thickness (d), void fraction (f[SUB&upsi;), and roughness layer (σ) as a function of H[SUB2]S concentration. The physical interpretation of the five modeling parameters obtained in the amorphous dispersion model applied to the case of n-C:S thin films is discussed. The Raman and ellipsometry results indicate that the average size of nanocrystallites in the sulfur-incorporated carbon thin films becomes smaller with increasing H[SUB2]S concentration, consistent with atomic force microscopy measurements where the distribution of grain size yielded a gamma around 20 nm. The band gap was found to decrease systematically with increasing H[SUB2]S concentration, indicating an enhancement of π-bonded carbon (sp[SUP2]C), in agreement... [ABSTRACT FROM AUTHOR]

Microcrystalline diamond and nanocrystalline carbon thin films prepared by microwave plasma assisted chemical vapor deposition were submitted to gamma radiation in order to study the radiation-induced structural transformation and the corresponding changes in the electron field emission properties. Electron field emission measurements and microstructural characterizations, which included Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy, were performed before and after gamma radiation doses of 1, 5, and 20 Mrads. Microcrystalline diamond showed a dramatic improvement in the emission properties only after a cumulative dose of 26 Mrad, while nanocrystalline carbon showed a relatively small but systematic decrease in turn-on field from 9 to 4 V/µm with increasing gamma radiation dose. The enhancement in emission characteristics associated with the radiation-induced microstructural transformation shows the critical role of defects with their associated electronic defect states, and of sp²-bonded carbon channels in the electron field emission mechanism of nanocomposite carbon materials. The results also indicate that nanocrystalline carbon tends to reach a state of damage saturation when submitted to Mrad doses of gamma radiation, suggesting the possibility of fabricating radiation buffer materials that would undergo internal sp[sup 3] - sp² interconversions while absorbing ionizing radiation without changing their average microstructure, which can be employed for preventing radiation from reaching sensitive materials. [ABSTRACT FROM AUTHOR]

Results are reported on the electron field emission properties of sulfur (S)-incorporated nanocrystalline carbon (n-C:S) thin films grown on molybdenum (Mo) substrates by hot-filament chemical vapor deposition (HFCVD) technique. In addition to the conventionally used methane (CH[sub 4]) as carbon precursor with high hydrogen (H[sub 2]) dilution, hydrogen sulfide-hydrogen (H[sub 2]S)/H[sub 2] premix gas was used for sulfur incorporation. The field emission properties for the S-incorporated films were investigated systematically as a function of substrate temperature (T[sub s]) and sulfur concentration. Lowest turn-on field achieved was observed at around 4.0 V/µm for the n-C:S sample grown at T[sub s] of 900 deg;C with 500 ppm of H[sub 2]S. These results are compared with those films grown without sulfur (n-C) at a particular T[sub s]. The turn-on field was found to be almost half for the S-assisted film thus demonstrating the effect of sulfur addition to the chemical vapor deposition process. An inverse relation between turn-on field (E[sub C]), growth temperature and sulfur concentration was found. The s incorporation also causes significant microstructural changes, as characterized with non-destructive complementary ex situ techniques: scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy (RS). S-assisted films show relatively smoother and finer-grained surfaces than those grown without it. These findings are discussed in terms of the dual role of sulfur in enhancing the field emission properties by controlling the sp² C cluster size and introducing substantial structural defects through its incorporation. The in-plane correlation length (L[sub a]) of sp² C cluster was determined from the intensity ratio of the D- and G-bands [I(D)/I(G)] in the visible RS as a function of deposition temperature and sulfur concentration using a phenomenological model. The turn-on field was found to decrease with increasing... [ABSTRACT FROM AUTHOR]

The article explains the optimization and preparation of the various perpendicular magnetic tunnel junctions with different Co-based multilayers. The various effects of Co thickness and the number of bilayers on the anisotropy of the junctions are also determined.

A tunable spatio-temporal Talbot imaging phenomenon is described, which is based on the radiation properties of an array of beam-steered metamaterial composite right/left-handed leaky-wave antennas. Numerical simulations have confirmed the self-imaging and pulse multiplication effects and their tunability capabilities as a function of frequency.

The thermal expansion of a number of select [M.sub.n+1]A[X.sub.n] (MAX) phases is measured. A good relation is found between the thermal expansion anisotropy and the elastic constant [c.sub.13] and the anisotropy in thermal expansion is related to the bonding between the M-A elements.

The ion transport process is investigated and the concomitant electrochemical tuning of Fermi level in single-wall carbon nanotube is determined employing in situ Raman spectroscopy technique. The Fermi level shift is estimated using mathematical modeling of Raman intensity and it is ~0.12 eV as an upper bound, affecting the resonance condition.

The heat capacitances [c.sub.p] of bulk polycrystalline samples of [Ti.sub.2]AlC, [V.sub.2]AlC and [Cr.sub.2]AlC in the 3-260 K temperature range are reported. The Debye temperatures are reasonably high and a linear dependence is found to be between the number of d electrons along the row Ti, and Cr and the density of states at the Fermi level.

The transition of the internal structure in microwave chemical-vapor-deposited carbon nanotubes (CNT) is investigated using scanning electron microscopy and high-resolution transmission electron microscopy. A growth model based on surface and bulk diffusions of carbon into and/or onto the Fe catalyst surface is used to describe the hollow to bamboo transition in nanotubes.

Investigations on the electron field-emission properties of as-grown, etched, and regrown carbon nanotube thin films were conducted. The results showed that the as-grown sample had a few strong emission spots and relatively high emission current density (~20 microA/square-cm at 1V/micrometer), while the regrown sample exhibited a significantly increased emission.

Presents a study which examined the optical properties of indium gallium arsenide and indium aluminum arsenide multiple quantum wells. Growth of compounds from molecular beam epitaxy; Measurement of the compounds' photoluminescence; Application of a picosecond-time-correlated photoluminescence spectroscopy.