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This article reports the superparamagnetic behavior of metal-free nitrogen-doped graphene quantum dots (N-GQDs). The pulsed laser ablation (PLA) method was utilized to synthesize N-GQDs with an average diameter of 3.45 nm and a high doping level (N/C) of 1.4. Magnetic properties of as-synthesized N-GQDs were explored by performing magnetization vs magnetic field (M–H) and magnetization vs temperature (M–T) measurements. M–H plots measured in a temperature range of 2–300 K revealed the superparamagnetic behavior of N-GQDs. The value of saturation magnetization was found to be directly correlated to nitrogen concentration and a saturation magnetization up to 28.7 emu/g was obtained at room temperature (300 K). M–T measurements with zero-field-cooled (ZFC) and field-cooled (FC) conditions were employed to study anisotropy energy barriers and blocking temperature. A variation in the blocking temperature (TB) from 288 to 61 K was observed when the external magnetic field (H) was changed from 0.1 to 0.6 T. The origin of superparamagnetism was attributed to the presence of graphitic nitrogen bonding configuration and defect states. The observed superparamagnetic properties along with the optical properties of N-GQDs create an opportunity for developing materials for biomedical applications and data recording devices. [ABSTRACT FROM AUTHOR]

Lithium–sulfur batteries (LiSBs) offer high energy density, cost-effectiveness, and eco-friendliness, making them promising for future energy storage. This study explores using BiFeO3 (BFO) nanoparticles (NPs) to tackle challenges such as lithium polysulfides (LiPs) and shuttle issues in LiSBs. It employs a solid-state melt diffusion technique, encapsulates sulfur in single-walled carbon nanotubes (SCNTs), and utilizes BFO for effective polysulfide control. Herein, composite cathodes of sulfur (S)/SCNTs (abbr. SCNT) were fabricated, and cells were designed using a BFO-coated separator (SCNT-BFS). In addition, a cathode modification was performed with composite S/SCNTs/BFO (SCNT-BF), and a comparative analysis was conducted to assess the effectiveness of the BFO in the separator and the cathode. Cyclic voltammetry measurements revealed that the increased current peak intensity at lower reduction potential in SCNT-BF and SCNT-BFS indicated control of higher-order LiPs (Li2Sx, where 4 ≤ x ≤ 8), resulting in the generation of more stable lower-order products (Li2S2/Li2S). The charge/discharge analysis revealed controlled LiPs, resulting in high-capacity retention in SCNT-BF (∼75%) and SCNT-BFS (∼88%) over 200 cycles, which yielded capacities of 526 and 700 mAh/g at C/8 (1C = 1675 mA/g). These promising results suggest that incorporating BFO into the cathode and separator can advance the commercialization of durable LiSBs. [ABSTRACT FROM AUTHOR]

This work demonstrates TiO2/β-Ga2O3 metal–dielectric–semiconductor (MDS) diodes with an average breakdown field beyond the material limits of SiC and GaN. These MDS diodes have lower conduction losses and higher breakdown voltage (Vbk) than the cofabricated Schottky barrier diodes (SBDs), simultaneously improving both on- and off-state parameters that are typically in competition with each other. With further optimized field management using p-NiO guard rings (GRs), the Ni/TiO2/β-Ga2O3 MDS diodes present a path to realistically utilize the high critical field of Ga2O3 without large forward conduction losses from a high-barrier junction. MDS diodes showed a lower Von (0.8 V) than the SBDs (1.1 V) from linear extrapolation of the current density-voltage (J-V) curve. The MDS diode had higher Vbk of 1190 V (3.0 MV/cm) compared to 685 V (2.3 MV/cm) for the SBD, and the MDS diode with the p-NiO guard ring saw further improvement with Vbk of 1776 V (3.7 MV/cm) compared to 826 V (2.5 MV/cm) for GR SBD. The BFOM (Vbk2/Ron,sp) of 518 MW/cm2 for the GR HJD is competitive with other literature results. A new figure of merit that includes the impact of turn on voltage is also proposed and demonstrated in this paper, which highlights how diodes perform in a practical high-power operation. This full paper is derived from the proceeding abstract of Willams et al. [IEEE Device Research Conference, Santa Barbara, CA, 25-28 June 2023 (IEEE, New York, 2023)]. [ABSTRACT FROM AUTHOR]

In this manuscript, we are reporting the role of ferroelectric nanoparticles (FNPs) coated separators in capacity retention at high current density (200 mA/g) on S/SWCNT composite cathodes. The ferroelectric nanoparticles of Bi0.925Gd.075Fe0.95Ni0.05O3 (BGFNO) of spontaneous polarization 2.5 µC/cm2 were coated on commercial separators (PP) as well as doped in sulfur/SWCNT composite cathodes. Our results show a discharge capacity retention of 72% by using commercial separator polypropylene (PP); however, a capacity retention improvement of more than 100% was obtained for FNPs coated separator, which has been placed on the anode side as well as on the cathode side. It is observed that the FNPs coated separator due to spontaneous polarization on the anode and cathode side acts as a repulsive charge on the separator surface to retard polysulfide migration via electrostatic repulsion and protects the surface of lithium from dendrite formation due to which it gains high capacity retention as well as stability. The coated separator controls the transport of carrier ions and side reactions; however, the FNPs doped cathode acts as an absorption center for polysulfides to enhance the electrochemical performance of the cells. The significance of this study is to design an efficient methodology, which could protect electrodes from dendrite formation via coated separator and modified cathode with the FNPs via suppressing the polysulfide formation to achieve high capacity retention and electrochemical cycle stability. [ABSTRACT FROM AUTHOR]

We report controlled silicon doping of Ga2O3 grown in plasma-assisted molecular beam epitaxy. Adding an endplate to the Si effusion cell enables the control of the mobile carrier density, leading to over 5-orders of magnitude change in the electrical resistivity. Room temperature mobilities > 100 cm 2 / V s are achieved, with a peak value > 125 cm 2 / V s at a doping density of low- 10 17 / cm 3 . Temperature-dependent Hall effect measurements exhibit carrier freeze out for samples doped below the Mott criterion. A mobility of 390 cm 2 / V s is observed at 97 K. [ABSTRACT FROM AUTHOR]

We report the fabrication of bifunctional nanocomposites consisting of ferroelectric Ba0.7Sr0.3TiO3 (BST) and ferromagnetic La0.67Sr0.33MnO3 (LSMO) at different concentrations via a high-temperature solid state route. The structural, dielectric, electrical, magnetodielectric (MD), magnetoelectric (ME) and magnetic properties of BST/LSMO nanocomposites were systematically investigated over a wide range of temperatures and frequencies. The X-Ray Diffraction analyses reveal the nanocrystalline nature of the heterostructures, wherein both perovskite phases co-exist. No parasitic phases were observed. The study of the dielectric properties shows that the nanocomposites exhibit relaxor ferroelectric character, with ferroelectric-paraelectric phase transition temperatures around 287-292K that do not follow the Curie-Weiss law. The electrical measurements indicate that ac conductivities of the nanocomposites follow the Jonscher's universal power law, with activation energies of 0.42-0.63 eV based on Arrhenius-type behavior at high temperatures. The nanocomposites exhibit well-defined ferromagnetic hysteresis loops at room temperature (RT). The MD and ME measurements at RT indicate that BST/LSMO exhibits a nonlinear ME effect at low frequencies, with a threshold near 0.5T. The magnetocapacitance (MCp) measurements evidence a quadratic dependence on magnetic field, further confirming the multiferroic nature of BST/LSMO. The order of MCp was found to be ~7% per Tesla. The analysis of the MCp measurements indicates that one of the BST/LSMO compositions studied can be considered as a new multiferroic compound. [ABSTRACT FROM AUTHOR]

Nanocrystalline diamond (NCD) thin films offer applications in various fields, but the existing synthetic approaches are cumbersome and destructive. A major breakthrough has been achieved by our group in the direction of a non-destructive, scalable, and economic process of NCD thin-film fabrication. Here, we report a cheap precursor for the growth of nanocrystalline diamond in the form of paraffin wax. We show that NCD thin films can be fabricated on a copper support by using simple, commonplace paraffin wax under reaction conditions of Hot Filament Chemical Vapor Deposition (HFCVD). Surprisingly, even the presence of any catalyst or seeding that has been conventionally used in the state-of-the-art is not required. The structure of the obtained films was analyzed by scanning electron microscopy and transmission electron microscopy. Raman spectroscopy and electron energy-loss spectroscopy recorded at the carbon K-edge region confirm the presence of nanocrystalline diamond. The process is a significant step towards cost-effective and non-cumbersome fabrication of nanocrystalline diamond thin films for commercial production. [ABSTRACT FROM AUTHOR]

We investigate the stability of the orange band photoluminescence (PL) of bifunctional nanoparticles of Mn2+-doped ZnS at 5% (ZnS:0.05Mn). These 4-nm nanoparticles are synthesized via a one-step inorganic chemical route under ambient conditions. The phase, crystallinity, and morphology are analyzed via X-ray and electron diffractions and high-resolution electron microscopy. Based on the thermally activated carrier-transfer model, it is found that orange emission is rather stable at low temperatures and possesses thermal activation energy of ∼18 meV. The analysis of the PL decay curves suggests the coexistence of multiple lifetimes, that the shortening observed in PL lifetime is not due to the Mn2+ ions, and that the orange band decay is stable in temperature range from 10 K to 300 K. The measured M-H hysteresis loops demonstrate that ZnS:0.05Mn nanoparticles exhibit ferromagnetic ordering below 30 K, unlike its bulk counterpart. No magnetic field dependence of the Mn2+ PL intensity is observed up to 1 T. The stability of the PL signal when subject to an applied magnetic field is discussed. This study offers experimental evidence to test low-dimensional dilute magnetic semiconductor models and widens the range of applications of ZnS:Mn nanostructures beyond optoelectronics. [ABSTRACT FROM AUTHOR]

Free standing graphene-diamond hybrid films have been fabricated using saturated hydrocarbon polymers as seeding material by hot filament chemical vapor deposition technique. The films are characterized with x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). The XRD shows the characteristic diffraction peaks of both diamond and graphene. The Raman spectrum shows the characteristic band of diamond at 1332 cm-1 and D, G, and 2D bands of graphene at 1349, 1592, and 2687 cm-1, respectively. Both SEM and TEM depict the presence of diamond and graphene in the films. The EELS recorded in the carbon K-edge region also shows the signature peaks of diamond and graphene. The free standing hybrid films exhibit a remarkably low turn-on field of about 2.4 V/μm and a high emission current density of 0.1 mA/cm2. Furthermore, emission currents are stable over the period of 7 days. The superior field emission characteristics of the free standing graphene-diamond hybrid films are attributed to the heat sink capability of diamond and high electrical conductivity of graphene. [ABSTRACT FROM AUTHOR]

The room-temperature photosensitivity of sulfur-doped micro-, submicro-, and nano-crystalline diamond films synthesized by hot-filament chemical vapor deposition was studied. The structure and composition of these diamond materials were characterized by Raman spectroscopy, scanning electron microscopy, and x-ray diffraction. The ultraviolet (UV) sensitivity and response time were studied for the three types of diamond materials using a steady-state broad UV excitation source and two pulsed UV laser radiations. It was found that they have high sensitivity in the UV region (as high as 109 s-1 mV-1 range), a linear response in a broad spectral range below 320 nm, photocurrents around ∼10-5 A, and a short response time better than 100 ns, which is independent of fluency intensity. A phenomenological model was applied to help understand the role of defects and dopant concentration on the materials' photosensitivity. [ABSTRACT FROM AUTHOR]

In the ongoing quest for energy production by nonconventional methods, energy conversion by vacuum and solid-state thermionic emission devices is one of the potentially efficient pathways for converting thermal energy directly into electrical power. The realization of practical of thermionic energy conversion devices strongly depends on achieving low work function materials, which is thus far a limiting factor. In an attempt to develop a new low work function thermionic material, this work reports thermionic emission energy distributions (TEEDs) from nanocrystalline diamond (NCD) films in the temperature range from 700 to 900 °C that reveal a consistent effective work function of 3.3 eV. The NCD films also exhibit emission peaks corresponding to higher work functions as indicated by shifts in their energy position and relative intensity as a function of temperature. These shifts thus appear to be related to instabilities in the NCD’s surface chemistry. The analysis of these data yields information on the origin of the low effective work function of NCD. [ABSTRACT FROM AUTHOR]

The temporal current stability and the current fluctuations of the field emission current obtained from ultrananocrystalline diamond (UNCD) films were investigated. The films were synthesized by Ar-rich dc plasma assisted hot filament chemical vapor deposition. The field emission properties were correlated to the films’ composition (sp2 and sp3 bonded carbon volume fractions) and nanostructure. The compositional analysis was done by modeling the films’ spectroscopic ellipsometry [Ψ (E), Δ (E)] data by using a two layer structure and the Bruggeman effective medium approximation. The results indicate that the local field enhancement in UNCD films is related to their intrinsic morphology at the diamond-grain boundary-vacuum interface, and the sp3/sp2 carbon volume fraction plays a significant function on the short- and long-term current stabilities of field emitted electrons. [ABSTRACT FROM AUTHOR]

The surface topography of hydrogenated tetrahedral amorphous carbon (ta-C:H) is critical for various applications such as microelectromechanical devices, magnetic and optical storage devices, and medical implants. The surface topography of ta-C:H films deposited by distributed electron cyclotron resonance plasma from C2H2 gas precursor was investigated. The effects of pressure, together with ion flux and energy, are studied by atomic force microscopy in relation to the structural evolution of the films. The results are compared with the predictions of the Edward-Wilkinson model [Proc. R. Soc. London, Ser. A 44, 1039 (1966)] recently proposed to account for ta-C:H growth and with previous interpretations based on hypersonic shock waves. The random hillocks observed on the smooth surfaces of ta-C:H films deposited at high pressure are thought to result from the interference of high energy shock waves triggered by C4Hx+ ions that produce overlapping collision cascades and induce nonlinear effects. [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]

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[SUBυ), 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]

Nanocrystalline carbon thin films were deposited by hot-filament chemical vapor deposition using a 2% concentration of methane in hydrogen. The films were deposited on molybdenum substrates under various substrate biasing conditions. A positive bias produced a continuous flow of electrons from the filament onto the substrate, while a negative bias caused the substrate to be bombarded with positive ions. Films were also grown under no bias, for comparison. Differences in the electron field emission properties (turn-on fields and emitted currents) of these films were characterized. Correspondingly, microstructural differences were also studied, as characterized with atomic force microscopy and Raman spectroscopy. Films grown under electron bombardment showed lower turn-on fields, smoother surfaces, and smaller grains than those grown under ion bombardment or no bias. A correlation between the enhanced emission properties and the nanocrystalline carbon material produced by the low-energy particle bombardment was found through the parameters obtained using spectroscopic ellipsometry modeling. The results confirm the significant role of defects on the electron field emission mechanism. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the Raman spectra of thermally reduced and untreated Y- and Ca-stabilized cubic zirconia. Decrease in Raman activity observed in acoustic mode region upon reduction; Absorption edge of Y-stabilized zirconia.

Presents information on a study which investigated the Raman spectra of hydrogenated amorphous silicon films produced by the glow discharge and radio frequency sputtering deposition techniques. Methods; Results; Discussion.

Discusses the performance of electrical measurements in different Pb[(Mg1/3Nb2/3)0.9Ti0.1]O3 ceramics exhibiting a spontaneous relaxor to ferroelectric transition. Dependence of relaxor behavior and phase transition on lead vacancies content; Increase of ordered domains and reduction of interactions between polar clusters.

Presents a Raman scattering study of the ferroelectric ordering in rhombohedral and tetragonal lanthanum-modified lead zirconate titanate ceramics. Basic aspects of Raman scattering; Experimental details and results; Summary and conclusion.

We demonstrate the hybrid fabrication process of a graphene integrated highly transparent resistive random-access memory (TRRAM) device. The indium tin oxide (ITO)/Al2O3/graphene nonvolatile memory device possesses a high transmittance of >82% in the visible region (370-700 nm) and exhibits stable and non-symmetrical bipolar switching characteristics with considerably low set and reset voltages (<±1 V). The vertical two-terminal device shows an excellent resistive switching behavior with a high on-off ratio of ∼5 × 103. We also fabricated a ITO/Al2O3/Pt device and studied its switching characteristics for comparison and a better understanding of the ITO/Al2O3/graphene device characteristics. The conduction mechanisms in high and low resistance states were analyzed, and the observed polarity dependent resistive switching is explained based on electro-migration of oxygen ions. [ABSTRACT FROM AUTHOR]

High temperature synthesis methods produce defects in 1D nanomaterials, which ultimately limit their applications. We report here the synthesis of micro-scale boron nitride nanotubes (BNNT) at low substrate temperature (300 °C) using a pulsed CO2 laser deposition technique in the presence of catalyst. The electron microscopic analyses have shown the nanotubes distributed randomly on the surface of the substrate. The average diameter (∼0.25 μm) of a nanotube, which is the highest reported value to date, is estimated by SEM data and confirmed by TEM measurements. These nanotubes are promising for high response deep-UV photoluminescent devices. A detailed synthesis mechanism is presented and correlated with the experimental results. [ABSTRACT FROM AUTHOR]

We report the optical and electrical properties of chemically-doped bilayer graphene stack by tetracyanoethylene, a strong electron acceptor. The Tetracyanoethylene doping on the bilayer graphene via charge transfer was confirmed by Raman spectroscopy and Infrared Fourier transform spectroscopy. Doped graphene shows a significant increase in the sheet carrier concentration of up to 1.520×1013 cm-2 with a concomitant reduction of the sheet resistance down to 414.1 Ω/sq. The high optical transmittance (ca. 84%) in the visible region in combination with the low sheet resistance of the Tetracyanoethylene-doped bilayer graphene stack opens up the possibility of making transparent conducting electrodes for practical applications. [ABSTRACT FROM AUTHOR]

Nanostructured materials, such as carbon nanotubes, are excellent cold cathode emitters. Here, we report comparative field emission (FE) studies on topographically tailored few layer MoS2 films consisting of h0001i plane perpendicular (⊥) to c-axis (i.e., edge terminated vertically aligned) along with planar few layer and monolayer (1L) MoS2 films. FE measurements exhibited lower turn-on field Eto (defined as required applied electric field to emit current density of 10 µA/cm²) ~4.5 V/lm and higher current density ~1mA/cm², for edge terminated vertically aligned (ETVA) MoS2 films. However, Eto magnitude for planar few layer and 1L MoS2 films increased further to 5.7 and 11 V/µm, respectively, with one order decrease in emission current density. The observed differences in emission behavior, particularly for ETVA MoS2 is attributed to the high value of geometrical field enhancement factor (β), found to be ~1064, resulting from the large confinement of localized electric field at edge exposed nanograins. Emission behavior of planar few layers and 1L MoS2 films are explained under a two step emission mechanism. Our studies suggest that with further tailoring the microstructure of ultra thin ETVA MoS2 films would result in elegant FE properties. [ABSTRACT FROM AUTHOR]

We report our studies on the responsivity of sulfur-doped diamond films to ultraviolet radiation using two types of device configurations: the planar configuration with electrodes directly on the diamond surface, and the electron field emission configuration with a bias electrode suspended above the diamond surface. Diamond films of different grain sizes were employed: microcrystalline diamond, sub-microcrystalline diamond, and nanocrystalline diamond. The responsivity values of diamond films in the field emission configuration reached ~10mA/W at around 220 nm, which is ~40% higher than that of the planar configuration. These responsivity values of diamond films are comparable to those of commercially available photodiodes in the wavelength range of 210-300 nm, but with the advantage of being solar blind. The responsivity data were correlated with the bandgap structure of sulfur-doped diamond. [ABSTRACT FROM AUTHOR]

We report photovoltaic response of highly transparent graphene/BiFe0.95Si0.05O3 (BFSiO)/ITO/ glass derived from bottom-up spin coating technique. The device exhibits short-circuit-current (ISC 0.75 mA) with 1000 fold upsurge and open-circuit-voltage (VOC∼0.45 V) under standard AM 1.5 illumination through graphene. In combination, ISC of 0.63mA and VOC of 0.35V for same illumination through ITO, reveals the prospects of harvesting indoor light. Also, crystallographic structure, red shift in band gap, leakage behavior, and ferroelectric characteristics of BFSiO thin films are reported. Reproducible transient response of ISC and VOC with quick switching (<100 ms) for 20 consecutive cycles and stability (95%) over test period of 16 weeks signifies high endurance and retentivity, promising for building integrated self-powered windows. [ABSTRACT FROM AUTHOR]

We report unipolar resistive switching suitable for nonvolatile memory applications in polycrystalline BiFeO3 thin films in planar electrode configuration with nonoverlapping Set and Reset voltages, On/Off resistance ratio of ~104 and good data retention (verified for up to 3,000 s). We have also observed photovoltaic response in both high and low resistance states, where the photocurrent density was about three orders of magnitude higher in the low resistance state as compared to the high resistance state at an illumination power density of ~100 mW/cm². Resistive switching mechanisms in both resistance states of the planar device can be explained by using the conduction filament (thermo-chemical) model. [ABSTRACT FROM AUTHOR]

Doubly substituted [Bi0.9La0.1][Fe0.97Ta0.03]O3 (BLFTO) films were fabricated on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition. The ferroelectric photovoltaic properties of ZnO:Al/BLFTO/ Pt thin film capacitor structures were evaluated under white light illumination. The open circuit voltage and short circuit current density were observed to be ~0.20V and ~1.35 mA/cm², respectively. The band gap of the films was determined to be ~2.66 eV, slightly less than that of pure BiFeO3 (2.67 eV). The PV properties of BLFTO thin films were also studied for various pairs of planar electrodes in different directions in polycrystalline thin films. [ABSTRACT FROM AUTHOR]

We report the photovoltaic properties of doped ferroelectric [Bi0.9La0.1][Fe0.97Ti0.02Zr0.01]O3 (BLFTZO) thin films. Polycrystalline BLFTZO films were fabricated on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition technique. Al-doped ZnO transparent top electrodes complete the ZnO:Al/BLFTZO/Pt metal-ferroelectric-metal capacitor structures. BLFTZO showed switchable photoresponse in both polarities. The open circuit voltage (VOC) and short circuit current (JSC) were found to be ~0.022 V and ~650 μA/cm², respectively after positive poling, whereas significant difference in VOC ~ 0.018V and JSC ~ 700 μA/cm² was observed after negative poling. The observed switchable photocurrent and photovoltage responses are explained on the basis of polarization flipping in BLFTZO due to the applied poling field. [ABSTRACT FROM AUTHOR]

We report the switchable photovoltaic effects in graphene/BiFeO3/Pt heterostructures. Pure phase polycrystalline BiFeO3 films were deposited on Pt/TiO2/SiO2/Si substrates by pulse laser deposition. A bilayer graphene was transferred onto the BiFeO3 film which serves as transparent conducting electrodes. The heterostructures showed switchable photovoltaic effect depending on ferroelectric polarization directions indicating depolarization field induced separation of photogenerated carriers. The open circuit voltage (VOC) and short circuit current density (JSC) were measured to be ~ 110 mV, ~92 μA/cm² in positive polarity and similar values were obtained when the polarity was reversed. The JSC and VOC also showed rapid response (< 100 ms) as a function of light exposure time. [ABSTRACT FROM AUTHOR]

We report a remarkable photovoltaic effect in pulsed laser deposited multiferroic aurivillius phase Bi5FeTi3O15 (BFTO) thin films sandwiched between ZnO:Al transparent conductive oxide top electrode and SrRuO3 bottom electrode fabricated on amorphous fused silica substrates. The structural and micro structural properties of these films were analysed by X-ray diffraction and atomic force microscopy techniques. The films were showing a photo sensitive ferroelectric behaviour with a notable apparent polarization in the range of 10-15 μC/cm². These films also exhibited a switchable photoresponse and this parameter was observed to be sensitive to polarisation field and the polarization direction. The device shows a large ON/OFF photo current ratio with an open circuit voltage of 0.14 V. The photo response at zero bias of this BFTO based heterostructures showed rapid increase to a saturation value of 6 μA at zero bias. [ABSTRACT FROM AUTHOR]

SPHINX is a 6 MA, 1-μs Linear Transformer Driver (LTD) operated by the CEA Gramat (France) and primarily used for imploding Z-pinch loads for radiation effects studies. Among the options that are currently being evaluated to improve the generator performances are an upgrade to a 20 MA, 1-μs LTD machine and various power amplification schemes, including a compact Dynamic Load Current Multiplier (DLCM). A method for performing magnetic ramp compression experiments, without modifying the generator operation scheme, was developed using the DLCM to shape the initial current pulse in order to obtain the desired load current profile. In this paper, we discuss the overall configuration that was selected for these experiments, including the choice of a coaxial cylindrical geometry for the load and its return current electrode. We present both 3-D Magneto-hydrodynamic and 1D Lagrangian hydrodynamic simulations which helped guide the design of the experimental configuration. Initial results obtained over a set of experiments on an aluminium cylindrical liner, ramp-compressed to a peak pressure of 23 GPa, are presented and analyzed. Details of the electrical and laser Doppler interferometer setups used to monitor and diagnose the ramp compression experiments are provided. In particular, the configuration used to field both homodyne and heterodyne velocimetry diagnostics in the reduced access available within the liner's interior is described. Current profiles measured at various critical locations across the system, particularly the load current, enabled a comprehensive tracking of the current circulation and demonstrate adequate pulse shaping by the DLCM. The liner inner free surface velocity measurements obtained from the heterodyne velocimeter agree with the hydrocode results obtained using the measured load current as the input. An extensive hydrodynamic analysis is carried out to examine information such as pressure and particle velocity history profiles or magnetic diffusion across the liner. The potential of the technique in terms of applications and achievable ramp pressure levels lies in the prospects for improving the DLCM efficiency through the use of a closing switch (currently under development), reducing the load dimensions and optimizing the diagnostics. [ABSTRACT FROM AUTHOR]

The temporal stability and electron field emission characteristics of simultaneously grown microcrystalline diamond and carbon nanostructures were investigated. The films were prepared by hot filament chemical vapor deposition using typical diamond deposition parameters, and iron oxide nanoparticles on the molybdenum substrate were employed as catalyst for their induction. The hybrid system presented better emission characteristics compared with microcrystalline diamond with turn-on fields as low as 2.45 V/μm (at 1 μA/cm2), and a current density of up to 0.24 mA/cm2 was achieved. The films showed relatively stable emission behavior for a period of 24 h. [ABSTRACT FROM AUTHOR]

The Sphinx machine [F. Lassalle et al., “Status on the SPHINX machine based on the 1microsecond LTD technology”] based on microsecond linear transformer driver (LTD) technology is used to implode an aluminium wire array with an outer diameter up to 140 mm and maximum current from 3.5 to 5 MA. 700 to 800 ns implosion Z-pinch experiments are performed on this driver essentially with aluminium. Best results obtained before the improvement described in this paper were 1–3 TW radial total power, 100–300 kJ total yield, and 20–30 kJ energy above 1 keV. An auxiliary generator was added to the Sphinx machine in order to allow a multi microsecond current to be injected through the wire array load before the start of the main current. Amplitude and duration of this current prepulse are adjustable, with maxima ∼10 kA and 50 μs. This prepulse dramatically changes the ablation phase leading to an improvement of the axial homogeneity of both the implosion and the final radiating column. Total power was multiplied by a factor of 6, total yield by a factor of 2.5 with a reproducible behavior. This paper presents experimental results, magnetohydrodynamic simulations, and analysis of the effect of such a long current prepulse. [ABSTRACT FROM AUTHOR]

The effects of adsorbates on field emission reproducibility from sulfur-incorporated nanocomposite carbon films grown by hot filament chemical vapor deposition were studied. Adsorption was induced on the films by liquid treatment at standard conditions using polar (1-butanol), moderately polar (ethyl acetate), and nonpolar (hexane) liquids. Adsorbates caused an increase in the turn-on field that was found to depend on the polarity of the liquid used: the larger (smaller) the polarity, the smaller (larger) the increase in turn-on field. The analysis of the data indicates that the increase in turn-on field is due to an increase in work function caused by adsorbates. Moreover, the hysteresis behavior, present in the field emission measurements, changes from clockwise to counterclockwise due to the adsorbates. The results are discussed in terms of the affinity between the adsorbates and the films’ surface and the adsorption-desorption process occurring on the films’ surface during emission. [ABSTRACT FROM AUTHOR]

The electron field emission properties of multiwall carbon nanotubes (MWCNTs) grown on sulfur-incorporated nanocomposite carbon (n-C:S) buffer layer were investigated. Both the MWCNTs and the n-C:S films were synthesized in a hot filament chemical vapor deposition system at relatively low methane concentrations. The n-C:S buffer layer provides good contact and adhesion to the Mo substrate and good contact and interface to the MWCNTs. The presence of this buffer layer was shown to improve the reproducibility and stability of the field emission behavior of MWCNTs. The turn-on field (EC) varies as much as 1.1 V/μm after high current density operation when there is no buffer layer, but variations up to only 0.3 V/μm are observed when the buffer layer is present. These results are interpreted in terms of the n-C:S buffer layer role, providing good adhesion and contact to the substrate side and to the MWCNTs, hence ensuring a high density of continuous paths for electrons from the substrate to the MWCNTs. [ABSTRACT FROM AUTHOR]

Subglottal pressure is one of the main voice control factors, controlling vocal loudness. In this investigation the effects of subglottal pressure variation on the voice source in untrained female and male voices phonating at a low, a middle, and a high fundamental frequency are analyzed. The subjects produced a series of /pae/ syllables at varied degrees of vocal loudness, attempting to keep pitch constant. Subglottal pressure was estimated from the oral pressure during the /p/ occlusion. Ten subglottal pressure values, approximately equidistantly spaced within the pressure range used, were identified, and the voice source of the vowels following these pressure values was analyzed by inverse filtering the airflow signal as captured by a Rothenberg mask. The maximum flow declination rate (MFDR) was found to increase linearly with subglottal pressure, but a given subglottal pressure produced lower values for female than for male voices. The closed quotient increased quickly with subglottal pressure at low pressures and slowly at high pressures, such that the relationship can be approximated by a power function. For a given subglottal pressure value, female voices reached lower values of closed quotient than male voices. 2005 Acoustical Society of America. [ABSTRACT FROM AUTHOR]