Effective electron emitters by molybdenum oxide-coated carbon nanotubes core–shell nanostructures

In this article, we showed that simple metal oxide coatings such as MoO₃ can be an effective enhancer for carbon nanotubes (CNTs) in field emission (FE) performance. For comparison, the FE properties of the pristine vertically aligned multi-walled CNTs with the metal oxide-coated CNTs were investigated. The metal oxide coating of the pristine CNTs was carried out by metal–organic chemical vapor deposition (MOCVD) method at 400 °C using Mo(CO)₆ as the precursor. The core–shell structure of the nanocomposite was studied by transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) results showed that the surface of the coating material was mainly MoO₃. FE test indicated that the MoO₃-coated CNTs film exhibited an enhanced performance than the pristine CNTs with a turn-on field of 1.33 V μm⁻¹ and a field enhancement factor β estimated to be ~7000. Ultraviolet photoelectron spectroscopy (UPS) results confirmed a lower electron emission barrier height for MoO₃-coated CNTs than for the pristine CNTs. The mechanism of the enhanced FE performance is discussed based on Schottky barrier effect.     

Differential formation of allophane and imogolite: experimental and molecular orbital study

Allophane and imogolite are naturally occurring aluminum silicate soil constituents with nano-ball and nano-tube morphology. Wall of the both materials is composed of Al(OH)₃ sheet with orthosilicic acid attached to it. Synthesis of allophane and imogolite can be controlled by addition of alkali and alkaline-earth metal ions. The main reaction product without or with small amounts addition of the metal ions is imogolite, while allophane forms when the metal ions were much added. The effect of metal ions on facilitating allophane formation and inhibition of imogolite formation were greater in the following order of Na, K < Ca, Mg. These metal ions affect the degree of dissociation of Si–OH group of orthosilicic acid, which may causes differential formation of allophane and imogolite. Structure optimization of the proto-imogolite model, precursor of allophane and imogolite, showed that when the Si–OH was undissociated, the shape of proto-imogolite model was transformed to asymmetrical in molecular configuration. This caused curling of the proto-imogolite model, which lead to formation of imogolite tube. On the other hand, when the Si–OH was dissociated, the shape of the proto-imogolite model was transformed to symmetrical configuration. This model curved to make a hollow sphere with placing the orthosilicic acid inside the sphere (allophane). Both of the experimental and molecular orbital calculation results proved that the dissociation of the Si–OH has an important role during the differential formation of allophane and imogolite.     

Enhanced field emission from the ZnO nanowires by hydrogen gas exposure

ZnO nanowires (ZNWs) were synthesized on Co-coated Si wafer via a carbon thermal reduction vapor transport method. Scanning electron microscopy, X-ray diffraction and transmission electron microscopy investigations show that these ZNWs present a high-quality single-crystalline hexagonal structure. Field emission (FE) characteristics of the ZNWs film were measured. A low turn-on voltage for driving a current density of 0.1 μA/cm² is about 3.9 V/μm. The field enhancement factor was determined to be ∼ 1180 for ZNWs film. Exposure of H₂ during FE causes a permanent increase in the FE current and a decrease in the turn-on field. Also, the field enhancement factor γ was finally increased from 1180 ± 20 to 1510 ± 20 after FE saturation.     

Electron field emission from the semimetallic TiO2 nanotube arrays

To enhance field-emission (FE) properties, as-prepared TiO₂ nanotube arrays (TNAs) successfully transformed from semiconductor into semimetal by a carbonization treatment under an argon/acetylene flux at 750 °C. After semimetallic transformation, the TNAs maintain tubular morphology, however, their surface become rougher. Especially, their electronic structures are significantly improved. A low turn-on field of 3.0 V/μm and a threshold field of 7.0 V/μm are obtained. Such improvements of FE properties can be mainly attributed to the enhancements of electrical property and rough surface morphology.     

Preparation and field emission properties of titanium polysulfide nanobelt films

TiS₃ nanobelt films, with widths of about 0.1–12 μm, thickness of about 20–250 nm, and lengths of up to 200 μm, have been grown on Ti substrates by a surface-assisted chemical-vapor-transport at 450 °C for 8 h. The TiS₃ nanobelt films were converted into TiS_1.71 nanobelt films by pyrolysis in a vacuum at 600 °C for 2 h. The work functions of the two films were determined by ultraviolet photoelectron spectroscopy measurements to be 4.60 and 4.44 eV, respectively. Preliminary field emission experiments using the nanostructures as cold electron cathodes showed that both materials gave significant emission currents. The turn-on fields (defined as the electric field required to produce a current density of 10 μA/cm₂) were about 1.0 and 0.9 V/μm, respectively, whereas the threshold fields (defined as the electric field required to produce a current density of 1 mA/cm₂) were about 5.6 and 4.0 V/μm, respectively. These data reveal that both materials have potential applications in field emission devices. This article is published with open access at Springerlink.com     

Field emission from the structure of well-aligned TiO2/Ti nanotube arrays

Well-aligned TiO₂/Ti nanotube arrays were synthesized by anodic oxidation of titanium foil in 0.5 wt.% HF in various anodization voltages. The images of filed emission scanning electron microscopy indicate that the nanotubes structure parameters, such as diameter, wall thickness and density, can be controlled by adjusting the anodization voltage. The peaks at 25.3° and 48.0° of X-ray diffraction pattern illuminate that the TiO₂ nanotube arrays annealed at 500 °C are mainly in anatase phase. The filed emission (FE) properties of the samples were investigated. A turn-on electric field 7.8 V/µm, a field enhancement factors approximately 870 and a highest FE current density 3.4 mA/cm² were obtained. The emission current (2.3 mA/cm² at 18.8 V/µm) was quite stable within 480 min. The results show that the FE properties of TiO₂/Ti have much relation to the structure parameters.     

Optical and field emission characteristics of anodic aluminium oxide/ZnO hybrid nanostructure

Arrays of ZnO nanowires (NWs) were fabricated within the well-distributed pores of anodic aluminium oxide (AAO) template by a simple chemical method. The photoluminescence (PL) and field emission (FE) properties of the AAO/ZnO NWs hybrid structure were investigated in detail. The hybrid nanostructure exhibits interesting PL characteristics. ZnO NWs exhibit UV emission at 378 nm and two prominent blue-green emissions at about 462 and 508 nm. Intense blue emission from the AAO template itself was observed at around 430 nm. Herein, for the first time we report the FE characteristics of the ZnO/AAO hybrid structure to show the influence of the AAO template on the FE property of the hybrid structure. It is found that the turn-on electric field of the vertically grown and aligned ZnO NWs within the pores of AAO template is lower than the entangled unaligned ZnO NWs extracted from the template. Although the AAO template exhibits no FE current but it helps to achieve better FE property of the ZnO NWs through better alignment. The turn-on electric field of aligned NWs was found to be 3 V μm⁻¹ at a current of 0.1 μA. Results indicate that the AAO embedded ZnO NW hybrid structure may find useful applications in luminescent and field emission display devices.     

Local Electronic Structure, O–T Phase Transition and Superconductivity in the Ba-site Nd-substituted YBCO System

A series of YBa_2-xNd_xCu₃O_y (x = 0–0.4) samples have been systematically studied by means of X-ray diffraction, transport property measurements and positron annihilation technology. The positron lifetime parameters show strong Nd substitution dependence. There is an obvious change of positron lifetime parameters around the O–T phase transition. The local electron density n_e and vacancy concentration C_v as a function of x were calculated from the positron lifetime results. The correlations between local electronic structure, O–T phase transition and superconductivity are discussed. The results confirmed that n_e mainly has an effect on high-T_c superconductivity by affecting the charge transfer between CuO₂ planes and Cu–O chains region or Ba–O layer. The vacancy properties in the orthorhombic phase and tetragonal phase are two intrinsic different types. Positron lifetime is very sensitive to the O–T phase transition in the YBCO systems that can be used as a useful technique to determine the O–T phase transition in these systems.     

Single crystal TiO2 nanorods: Large-scale synthesis and field emission

A facile growth procedure of low-pressure chemical vapor deposition has been developed to synthesize single crystal TiO₂ nanorods on Ni-coated Ti substrate. The results indicate that the as-prepared nanorods are high purity single crystal with a [110] preferential orientation. Using directly the nanorods as emitters, their field emission (FE) properties have been investigated. The turn-on field of the nanorods decreases from 3.96 V/μm to 2.98, and 2.16 V/μm as the growth temperature increases from 700 °C to 750, and 800 °C, respectively. In addition, the nanorods show good FE stability during 480 min. Such behavior is mainly related to the morphology of nanorods, which depends on the growth temperature. Also, the growth mechanisms of the nanorods are analyzed in detail based on the experimental results.     

Synthesis of CeB6 thin films by physical vapor deposition and their field emission investigations

High quality CeB₆ thin films have been obtained through direct evaporation of raw micron-sized CeB₆ powders at a pressure of 70 Pa. The X-ray diffraction (XRD), Raman spectrum, scanning electron microscope (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and the field-emission equipment were used to characterize the morphology, structure, composition and FE properties of the samples. The XRD and Raman spectrum analysis results show the as-prepared product is cubic phase CeB₆. The TEM, SAED and HRTEM analysis reveal that the samples are mixtures of thin films (polycrystalline) and small crystals (single crystallines aligned preferentially in the [1 1 0] direction). Compared to oxide nanostructures, field-emission measurements show that the CeB₆ films have better FE performance with turn-on field and threshold field of 12.93 V/μm and 14.86 V/μm, respectively.     

Effect of Pb (Yb1/2Nb1/2)O3 on phase transition and thermal and electrical properties of PZ–PYbN solid solution on PZ-rich side

The (1 − x)PbZrO₃–xPb(Yb_1/2Nb_1/2)O₃ (PZ–PYbN) ceramics, with the compositions, x = 0.00–0.50, were prepared by the wolframite precursor method. The crystal structure and electrical and thermal properties of PbZrO₃ ceramic were investigated as a function of the composition, x, using X-ray diffraction, dielectric spectroscopy, hysteresis measurement and differential scanning calorimetry techniques. The results indicated that the solid solution, PZ–PYbN, changed from orthorhombic to rhombohedral symmetry when the amount of PYbN increased. The pyrochlore phase identified as Yb/Nb mixed compound was observed at the composition, x ≥ 0.2. For the compositions, x = 0.00–0.10, ceramics showed a sharp phase transition from AFE to PE. Furthermore, the intermediate FE phase was absent from the PZ–PYbN system.     

Preparation of grass-like GaN nanostructures: Its PL and excellent field emission properties

We here report highly pure and single crystalline grass-like gallium nitride (GaN) nanostructures obtained on silicon substrate via catalyst-assisted CVD route under NH₃ atmosphere inside horizontal tube furnace (HTF) by pre-treating the precursors with aqueous NH₃. The as-obtained GaN nanostructures were characterized by XRD, SEM, EDS, HRTEM and SAED. The field emission (FE) characteristics of grass-like GaN nanostructures exhibited a turn-on field of 7.82 V μm^− 1 and a threshold field of 8.96 V μm^− 1 which are quite reasonable for applications in electron emission devices, field emission displays and vacuum microelectronic devices. Room temperature photoluminescence (PL) measurements of grass-like GaN nanostructures exhibited a strong near-band-edge emission at 368.8 nm (3.36 eV) without any defects related emissions which shows its potential applications in optoelectronics.     

Influence of Zr-substitution on phase transitions character in polycrystalline Ba(Ti<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)O<Subscript>3</Subscript>

The results of X-ray diffraction, scanning electron microscopy, and dielectric measurements performed for polycrystalline Ba(Ti_1−x Zr _x )O₃ are presented. Data from these measurements show a change of the type of paraelectric–ferroelectric (PE–FE) phase transition (PT) from sharp PT (x < 0.10), through diffuse phase transition (0.10 ≤ x ≤ 0.20), to relaxor type one (x = 0.30). The lack of structural PT at the temperature T _m (the temperature of the maximum of real part of the electric permittivity ε′) for 0.10 ≤ x ≤ 0.30, suggest that this PE–FE transition is not connected with the change of crystal structure. A phase angle of about Φ ≈ −90°, between electric current and applied voltage, suggests the occurrence of polar regions (clusters) below 400 K. The change of these clusters polarizability and their lability at the T _m is postulated as possible origin of the observed maximum of ε′.     

Simulation of the oxidation kinetics of NOx in a gas discharge

We investigate the mechanisms of the oxidation of NO in a gas discharge. We consider the influence of admixed water on the efficiency of removal of nitrogen oxides. Power expenditures are evaluated and optimal values for the temperature of a mixture and lifetime of particles in a reactor are found. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 71, No. 2, pp. 299–310, March–April, 1998.     

Effects of La occupation site on the dielectric and piezoelectric properties of [Bi0.5(Na0.75K0.15Li0.10)0.5]TiO3 ceramics

The crystal structure, phase transition and ferroelectric (FE)/piezoelectric properties were investigated for three types of La-doped [Bi_0.5(Na_0.75K_0.15Li_0.10)_0.5]TiO₃ ceramics. The dielectric measurements showed that the transition between FE and antiferroelectric (AFE) phases near 180 °C became pronounced by La addition, and the maximum permittivity was observed at 360 °C in La-doped samples, whereas at 290 °C in non-doped samples. Normal FE and excellent piezoelectric properties were observed by P–E hysteresis loop and piezoelectric measurements in samples without vacancy. However, when the A-site or B-site vacancies were formed, the temperature range of AFE phase extended even appeared at room temperature, which resulted in the presence of deformed P–E curves and decrease of piezoelectric properties. It was suggested that the AFE phase originated from the decoupling effect between BO₆ octahedra in ABO₃ perovskites due to the A-site and/or B-site vacancies.     

Complementary Analysis of the Field Perturbation Theory of Superconductivity

We reexamine the Nambu–Gorkov perturbation theory of superconductivity. We suggest that any field perturbation theory of superconductivity should be based on the Bogoliubov–Valatin (BV) quasi-particles. We show that two such different fields (and two additional analogous fields) may be constructed on the basis of this suggestion. The Nambu field is only one of them. For the field that is different than Nambu’s, the coherence field, the interaction is given by means of two interaction vertices that are based on the Pauli matrices τ₁ and τ₃. Consequently, the Hartree integral for the off-diagonal pairing self-energy may be finite, and in some cases large. We interpret the results in terms of conventional superconductivity and also discuss briefly the possible implications to HTSC.     

Nonequilibrium processes in Hg1−x CdxTe n +−p junctions in a magnetic field

The results of measurements of the relaxation and current-voltage characteristics of Hg_{1− x} Cd_xTen ⁺−p junctions in a magnetic field are presented. It is shown that the lifetime of the nonequilibrium electrons in the p-type region undergoes an increase in a magnetic field, which can be associated with the heterogeneous distribution of defects from the junction boundary. The current-voltage characteristics in a magnetic field exhibit suppression of the diffusion component of the current and an increase in the contribution of the generation-recombination channel, as well as the appearance of shunting channels, which are associated with the influence of the surface. Pis’ma Zh. Tekh. Fiz. 23, 88–94 (October 26, 1997)     

Enhanced field electron emission of graphene sheets by CsI coating after electrophoretic deposition

Because of the large quantities of edges, graphene can serve as an efficient edge emitter for field emission (FE). Cesium iodide (CsI) coating was promising to enhance the electron emission and utilized in FE applications. In this work, FE of graphene sheets after electrophoretic deposition (ED) was studied. Electron emission property of GS was obviously improved by coating with CsI. The turn-on field of GS decreased from 4.4 to 2.5 V/ μm; and threshold field decreased from 9 to 5.8 V/μm, respectively. This FE improvement must due to a higher effective density of emission site generated around the GS surface after coating. Scanning electron microscopy (SEM) and computation were taken to reveal the influence after coating. Investigations of CsI coated MWCNTs were also compared in order to better understand the origin of the low turn-on electric field obtained by GS.     

Elastic Properties of Vanadium Pentoxide Aggregates and Topological Defects

We study the aqueous solution of vanadium pentoxide by using topology methods. The experiments by Zocher, Kaznacheev, and Dogic exhibited that in the sol phases of V₂O₅–H₂O, the tactoid droplets of V₂O₅ can coalesce. In the magnetic field, this effect is associated with a gauge field action, viz. we consider coalescence (“junction,” in the topologically more convenient term) of droplets as annihilation of topological defects, concerning with the tactoid geometry. We have shown, that in the magnetic field, the tactoid junction is mainly caused by non-Abelian monopoles (vortons), whereas the Abelian defects almost do not annihilate. Taking into account this annihilation mechanism, the estimations of time-aging of the V₂O₅–H₂O sols may be specified.     

The emission characteristics of low-pressure water vapor discharge UV emitters

We have studied the characteristics of UV emission sources operating on low-pressure normal (H₂O) and heavy (D₂O) water vapor excited by periodic-pulsed and glow discharges. The emission in a 300–330 nm wavelength interval has been studied in detail for water vapor pressures ranging from 50 to 2500 Pa. A comparison of the characteristics of emission from discharge plasma at low (50–150 Pa) and elevated (2.0–2.5 kPa) water vapor pressures reveals significant differences in the character of emission spectra, which can be related to the different types of emitting species (hydroxy radicals versus small clusters of such radicals and water molecules). Discharge current and emission intensity pulses in the periodic-pulsed discharge regime have been measured.