Improving the field-emission properties of carbon nanotubes by magnetically controlled nickel-electroplating treatment

A novel magnetically controlled Ni-plating method has been developed to improve the field-emission properties of carbon nanotubes (CNTs). The effect of the magnetic field and Ni-electroplating on CNT field-emission properties was investigated, and the results are demonstrated using scanning electron microscopy, J-E and the duration test. After treatment, the turn-on electric field declines from 1.55 to 0.91 V/μm at an emission current density of 100 μA/cm², and the emission current density increases from 0.011 to 0.34 mA/cm² at an electric field of 1.0 V/μm. Both the brightness and uniformity of the CNT emission performance are improved after treatment.     

Electronic properties and field emission of carbon nanotube films treated by hydrogen plasma

Large-scale and very even multi-wall carbon nanotube (MWNT) films have been obtained at room temperature by an electrophoresis deposition technique. The characterization, by means of scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and micro-Raman spectroscopy, shows that the MWNTs with hydrogen-plasma (HP) treatment are covered by onion-like nanolumps, and three-dimensional multiple-way-connected nanotube webs are formed. The electronic property of the treated MWNT film is converted from semiconducting to metallic. The field-emission test indicates that the HP-treated MWNT film has a low threshold of 1.1 V/m (at 0.1 A/cm²), a high emission light spot density of about 10⁵ cm^-2, and a stable and suitable emission current. The conversion mechanism of the treated carbon nanotube structure and the reason for the change of the electronic and field-emission characteristics of the MWNT films are discussed. PACS 81.07.De; 82.33.Xj; 85.45.Db     

Temperature dependence of current-voltage characteristics of Sn/p-Si Schottky contacts

The current-voltage (I-V) characteristics of Sn/p-Si Schottky barrier diode have been measured over a wide range of temperature (80-300 K) and interpreted on the basis of thermionic emission mechanism by merging the concept of barrier inhomogeneities through a Gaussian distribution function. The analysis has revealed an anomalous decrease of apparent barrier height Φ_b0, increase of ideality factor n, and nonlinearity of the activation energy plot at lower temperatures. A Φ_b0 versus 1/T plot has been drawn to obtain evidence of a Gaussian distribution of barrier heights, and values of 0.97 eV and 0.084 V for the mean barrier height and standard deviation σ₀ have been obtained, respectively, from this plot. A modified ln(I₀/T²)−(q²σ₀²/2k²T²) versus 1/T plot gives and Richardson constant A** as 0.95 eV and 15.6 A cm⁻² K⁻², respectively. It can be concluded that the temperature dependent I-V characteristics of the Sn/p-Si Schottky barrier diode can be successfully explained on the basis of a thermionic emission mechanism with Gaussian distribution of the barrier heights. We have also discussed whether or not the junction current has been connected with thermionic field-emission mechanism.     

On the motional states of dipolar impurities in ionic matrices

Abstract

The effect of lattice vibrations on the librational frequencies and tunneling frequencies is considered for the case of alkali halides doped with diatomic impurities. It has been shown that this effect is important for the tunneling frequency while the librational frequency changes only slightly. The implications of this have been discussed in light of certain anomalous results observed in KCl-OH^? and KCl-CN-.     

大电流碳纳米管场发射阴极研究

报道了在较大发射面积上获得较大场发射电流的碳纳米管场发射阴极。为了加强场发射电流,在丝网印刷浆料中增加一种金属纳米颗粒,金属颗粒增强了碳纳米管发射体和衬底的接触,提高碳纳米管和衬底的粘附作用。利用改进后的丝网印刷方法制备了大电流碳纳米管场发射阴极,测得最大发射电流为68.0 mA,阴极有效发射面积约1.1 mm2,发射电流密度约6.2 A/cm2;并成功将改进方法制备的大电流场发射碳纳米管阴极应用于场发射真空器件原型。实验证明这种具有较大发射电流和较大发射电流密度的场发射能够满足部分大功率电子器件的需求。收稿日期:; 修订日期:     

Magnetic properties and energy transfer in the luminophors CaS:Eu,Cl

Studies have been performed on the magnetic properties and electron paramagnetic resonance (EPR) spectra of the small-crystal luminophors CaS:Eu,Cl. By comparing the EPR and magnetic susceptibility data we have determined that Eu enters into the CaS matrix primarily in the trivalent state Eu³⁺. We have found that the magnetic susceptibility of the sample with the lowest europium content (5×10⁻³ at. %) has a sharp peak at T≈5 K. A model is proposed of clusters into which the europium ions in these luminophors can associate. On the basis of this model an explanation is given for the anomalous temperature dependence of the magnetic susceptibility as well as a long list of other experimental facts (including peculiarities of the thermal luminescence). Zh. éksp. Teor. Fiz. 113, 1698–1707 (May 1998)     

Synthesis and field-emission of aligned SnO2 nanotubes arrays

Aligned tin dioxide (SnO₂) nanotubes have been synthesized by high-frequency inductive heating. Nanotubes with high yield were grown on silicon substrates in less than 5 min, using SnO₂ and graphite as the source powder. Scanning electron microscopy and transmission electron microscopy showed nanotube with diameters from 50 to 100 nm and lengths up to tens of mircrometers. The SnO₂ nanotubes synthesized under the optimum condition have better field-emission characteristics. The turn-on field needed to produce a current density of 10 μA/cm² is found to be 1.64 V/μm. The samples show good field-emission properties with a fairly stable emission current. This type of SnO₂ nanotubes can be applied as field emitters in displays as well as vacuum electric devices.     

Analysis of the current-transport mechanism across a CVD diamond/silicon interface

This work presents a study on the mechanism of injection and charge transport through a CVD diamond/n⁺-Si interface. The current-voltage-temperature characteristics of CVD diamond/silicon heterojunctions measured in the temperature range 119-400 K have been interpreted according to thermionic theory and thermionic field-emission theory. This junction shows deviations from the ideal thermionic theory current model, suggesting the presence of surface states, thin-layer depletion and/or non-homogeneity in the diamond/silicon interface. The T₀ anomaly has been used to explain the behaviour of the ideality factor with temperature. At very low temperatures tunnelling may occur because the E₀₀ values for these junctions are close to the value expected by thermionic field-emission theory. The usual activation-energy plot deviates from linearity at low temperatures. This deviation has been corrected supposing a ln(J_S/T²) versus 10³/nT plot. Under these conditions the Richardson constant is found to be 0.819 A cm⁻² K⁻², which is close to the theoretical value of 1.2 A cm⁻² K⁻². Field-emission device is a promising application for diamond/silicon structure.     

Mechanisms of EPR line broadening in Pb5Ge3O11 near a structural transition

The polar angular dependence of the anomalous linewidth of three EPR transitions in the Gd³⁺ trigonal center has been used to determine the broadening mechanism. It is shown that the dominant mechanism of Gd³⁺ EPR signal broadening in the vicinity of the ferroelectric transition is the critically growing spread of the b ₂₁ and c ₂₁ parameters accounting for the odd-order fields of remote charge-compensating defects. Fiz. Tverd. Tela (St. Petersburg) 40, 321–326 (February 1998)     

On the core concentration and the formation kinetics of thermal donors in silicon

Abstract

In this work the anomalous formation kinetics of old thermal donors (TD) observed in the initial stage of formation in pre-heat-treated carbon-rich samples are studied. Computer simulations applying a reaction scheme where the TD-complexes are subsequently formed by single oxygen diffusion reveal that the experimental results can only be acounted for if the pre-existing TD-core involves three oxygen atoms and has a concentration in the range 10¹³-10¹⁴ cm^?3. Carbon is found to have a major influence on the production rate of pre-existing cores.     

EPR of the ferroelectric phase transition in Li2Ge7O15:Cr3+ crystals

A Cr³⁺ EPR study of lithium heptagermanate crystals, Li₂Ge₇O₁₅ (LHG), close to the phase transition is reported. Orientation dependences of the spectra in the paraelectric phase of LHG have been measured. An anomalous broadening of the resonant lines accompanied by a crossover in their shape was found in the vicinity of the transition point. Doublet splitting of the EPR lines was observed to occur below T _C as a result of emergence of two structurally nonequivalent positions for Cr³⁺ ions. Fiz. Tverd. Tela (St. Petersburg) 40, 1102–1105 (June 1998)     

On low-field electron emission mechanisms

It is established by direct experiments that the main component of the stationary field-emission current in fields E<10⁵ V/cm is due to piezogeometric intensification (by a factor of 10³) of the electricfield at the end faces of piezoelectrically active films. An emission mechanism governed by electrons supplied by tunneling from the valence band of the piezoelectric is proposed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 823–827 (10 June 1997)     

Chiral perturbation theory for anomalous processes

A general expression containing all the divergences in Chiral Perturbation Theory at one loop for anomalous processes has been calculated. The nonrenormalization of some amplitudes (Adler-Bardeen theorem) is observed. The different types of corrections are identified. The contributions from vector meson exchange are determined for all anomalous processes. Explicit cancellation of divergences is shown for π⁰, η and η′ decays to γγ^*. The dependence on the γ^* mass squared is predicted to be essentially the same for all the three processes. It proceeds mainly through the VMD estimate of the finite parts of the next order Lagrangian and compares reasonably well with the available experimental data.     

Anomalous Hall effect in granular alloys

A theoretical study is performed of the anomalous Hall effect in granular alloys with giant magnetoresistance. The calculation is carried out within the Kubo formalism and the Green’s function method. The mechanism of asymmetric scattering of the spin-polarized current carriers is considered with allowance for a size effect associated with scattering not only by one grain, but also with more complicated processes of transport among two and three grains. It is shown that scattering of conduction electrons by the interfaces of the grains and the matrix has a substantial effect on the magnitude of the anomalous Hall effect and determines its sign. In general, correlation between the quantities ρ _H and ρ ² is absent, where ρ _H is the Hall resistivity and ρ is the total resistivity of the granular alloy. However, numerical calculation shows that for certain values of the model parameters ρ _H∼ρ ^3.8 and for these same parameter values the amplitude of the giant magnetoresistance reaches 40%, which is found to be in quantitative agreement with the experimental data for Co₂₀Ag₈₀ alloys [P. Xiong, G. Xiao, J. Q. Wang et al., Phys. Rev. Lett. 69, 3220 (1992)]. It is also shown that increasing the resistivity of the matrix leads to a significant growth in the anomalous Hall effect, and more substantial growth for alloys with small grain size, which is in good agreement with experiment [A. B. Pakhomov, X. Yan, and Y. Xu, J. Appl. Phys. 79, 6140 (1996); [X. N. Jing, N. Wang, and A. B. Pakhomov, Phys. Rev. B 53, 14032 (1996)]. Zh. éksp. Teor. Fiz. 112, 2198–2209 (December 1997)     

Kinetics of the initial stage in a first-order phase transformation in thin films

The nucleation and growth of islands of a new phase on the surface of solids has been studied both experimentally and theoretically for the particular case of the transition from the pyrochlore to perovskite phase in a thin film of a lead zirconate-titanate ferroelectric. This transformation was chosen because the new-phase islands have a stable circular shape in this case, a relatively large size (10⁻⁵–10⁻⁴ m) permitting their observation with an optical microscope, and a low growth rate (10⁻⁸–10⁻⁹ m/s). A theoretical analysis of the process, based on the kinetic theory of first-order phase transitions proposed earlier, has been carried out and the behavior in time of all main characteristics of a phase transformation, namely, nucleation rate, concentration of the new-phase islands, their size distribution, and relative overheating, has been calculated. The same characteristics have been measured experimentally, thus permitting one for the first time to make a thorough comparison of the theoretical with experimental data on the kinetics of first-order phase transitions. They have been found to be in a good agreement. Fiz. Tverd. Tela (St. Petersburg) 39, 121–126 (January 1997)     

Energy dependence of transition strengths extracted from 12C(τ, α) and 16O(τ, α)

The (τ, α) reaction on ¹²C and ¹⁶O has been studied at 24 and 28 MeV bombarding energy. Differential cross sections have been obtained and analysed with DWBA to yield relative spectroscopic factors. The comparison with results obtained at lower bombarding energies shows that the strengths of some anomalous shell-model-forbidden transitions decrease with energy. This agrees qualitatively with the prediction of a reaction model which assumes a cluster substitution mechanism to explain the anomalous (τ, α) transitions.     

Statistical description of the motion of dislocation kinks in a random field of impurities adsorbed by a dislocation

A model has been proposed for describing the influence of impurities adsorbed by dislocation cores on the mobility of dislocation kinks in materials with a high crystalline relief (Peierls barriers). The delay time spectrum of kinks at statistical fluctuations of the impurity density has been calculated for a sufficiently high energy of interaction between impurities and dislocations when the migration potential is not reduced to a random Gaussian potential. It has been shown that fluctuations in the impurity distribution substantially change the character of the migration of dislocation kinks due to the slow decrease in the probability of long delay times. The dependences of the position of the boundary of the dynamic phase transition to a sublinear drift of kinks x ∝ t^δ (δ σ 1) and the characteristics of the anomalous mobility on the physical parameters (stress, impurity concentration, experimental temperature, etc.) have been calculated.     

Application of mechanical scanning to ion implantation

Abstract

Atomic depth profiles from Be-implanted Si have been examined as a function of implant fluence and annealing, and the results have been correlated with theoretically calculated implantation induced damage profiles. The Be atomic depth profiles were obtained by secondary ion mass spectrometry (SIMS) techniques from samples implanted at 300 keV to fluences ranging from 2 × 10¹² to 10¹⁵ cm^?2. Subsequent to annealing at 600°C for 30 min, the Be SIMS profiles exhibited anomalous redistribution effects. The Be profiles obtained from the annealed samples had the same general features as the depth distribution of implant energy deposited into damage, based on Brice's¹ calculations. The correlation of the SIMS atomic profiles and the theoretical damage profiles indicated that Be “decorates” the implantation induced damage regions while redistributing during the annealing process.     

Field emission and photoluminescence of ZnO nanocombs

Three kinds of new comb-shape nanostructures of ZnO have been grown on single silicon substrates without catalyst-assisted thermal evaporation of Zn and active carbon powders. The morphology and structure of the prepared nanorods are determined on the basis of field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and x-ray diffraction (XRD). The growth mechanism of the ZnO nanocombs can be explained on the basis of the vapor–solid (VS) processes. In nanocombs 1 and nanocombs 2, the comb teeth grow along [0001] and the comb stem grows along [ $01\overline{1}0$ ], while in nanocombs 3, nanoteeth grow along [ $01\overline{1}0$ ] and stem grows along [0001]. The photoluminescence and field-emission properties of ZnO nanocombs 1–3 have been investigated. The turn-on electric field of ZnO nanocombs 1–3, which is defined as the field required to producing a current density of 10 μA/cm², is 9, 7.7 and 7.1 V/μm, respectively. The field-emission performance relies not only on the tip’s radius of curvature and field enhancement factor, but also on the factor evaluating the degree of the screening effect.     

Systematic row and zone axis STEM defect image simulations

Transmission electron microscopy (TEM) has been instrumental in advancing the field of crystalline defect analysis. Conventional TEM imaging techniques, such as bright field (BF), dark field (DF), and weak beam dark field (WBDF or g–3g) imaging, have been well-documented in the scientific literature, with simulation methods readily available for each. The present contribution highlights the use of a field-emission TEM, operated in scanning transmission electron microscopy mode, as a viable tool for defect analysis. Common techniques such as two-beam diffraction contrast and zone axis imaging are applied to defect analysis; both experimental and computational results are presented. Effects of experimental parameters such as camera length, beam divergence angle, and diffraction aperture placement are also discussed and illustrated by both experimental and computed micrographs of stacking faults.