S. Z. Deng

Needle-shaped silicon carbide nanowires: Synthesis and field electron emission properties

Bunches of needle-shaped silicon carbide (SiC) nanowires were grown from commercially available SiC powders in thermal evaporation process and using iron as catalyst. Their structure and chemical composition were studied by Raman spectroscopy and high-resolution transmission electron microscopy. The powder of these nanowires may be easily dispersed, and was used to form samples of field electron emitters. The needle shape of individual nanowires is well-suited to field electron emission. Stable emission with current density of 30.8 mA/cm2 was observed at fields as low as 9.6 V/μm, and current density of up to 83 mA/cm2 was recorded.

Field emission from crystalline copper sulphide nanowire arrays

Straight crystalline copper sulphide (Cu2S) nanowire arrays have been grown by using a simple gas–solid reaction at room temperature. These were demonstrated to exhibit semiconductor properties. Field emission was observed at a field of ∼6 MV/m, and its current-field characteristics deviate from Fowler–Nordheim theory, i.e., showing a nonlinear Fowler–Nordheim plot. The uniform emission from the whole arrays was observed using transparent anode technique, and their variation with applied field was recorded. The emission from individual nanowires was also studied using a field emission microscope, and was found to consist of a number of spatially resolved diffuse spots. Finally, stable emission current at different levels and over time was recorded. These findings indicate that semiconductor nanowires as cold cathode have a potential future, worthy of further comprehensive investigation. The technical importance of using semiconductor nanowires as cold cathode emitter is given.

Temperature dependence of field emission from cupric oxide nanobelt films

Films of aligned cupric oxide nanobelts have been prepared in an aqueous solution at room temperature. Field-emission characteristics, including emission-current–applied-field plot and emission site distribution, have been studied using the transparent anode technique. In addition, the temperature dependence of the field-emission characteristics has been studied from room temperature to 750 K. The threshold field for obtaining a current density of 10 μA/cm2 is ∼11 MV/m. This decreases with increasing temperature, and at 700 K it is ∼6 MV/m. At a fixed field of 10 MV/m, about a three-orders-of-magnitude increase of the emission current level has been observed. The results show that the cupric oxide nanobelt is a promising candidate for cathode material in a thermoelectric conversion device based on field emission.

GRAPHITIZATION OF NANODIAMOND POWDER ANNEALED IN ARGON AMBIENT

Nanodiamond powder was annealed at each of the following temperatures: 300, 600, 800, 1000, and 1150 °C, for an hour in flowing argon ambient. The variations of x-ray diffraction patterns and Raman spectra of the powder with different annealing temperatures were studied. While being annealed at temperatures higher than 800 °C, the powder can undergo a phase-transition process from cubic diamond to graphite. In addition, the size of nanodiamond crystallites decreased from ∼50 to ∼25 A. The physical mechanism responsible for the variation in Raman spectra is discussed using a phonon-confinement model.

Synthesis and field-emission properties of aligned MoO3 nanowires

Aligned MoO3 nanowires have been grown on silicon substrates without using any catalyst. They were prepared in a two-step process: first by thermal evaporation and then further processed by oxidation. The MoO3 nanowires are of crystalline and have an orthorhombic structure. They also have high purity. Field-emission measurement showed that, typically, their turn-on field and threshold field were about 3.5 and 7.65 MV/m, respectively. Furthermore, the spatial distribution of emission sites was studied using transparent anode technique and the emission current of the sites was relatively uniform. These may be attributed to very good uniformity in the height and diameter of the nanowires, and to the separation between nanowires. Finally, the stability of the emission current over time was found to be within 10%. These findings indicate that MoO3 nanowires as a cold cathode have a potential future.

Synthesis of crystalline alumina nanowires and nanotrees

Abstract Crystalline alumina nanowires were synthesized at elevated temperatures in a catalyst-assisted process using iron as catalyst. Nanotrees that formed by alumina nanowires were also found. SEM shows that typical nanowires are around 50 nm in diameter and around 2 μ m in length. The tree trunk of the nanotrees are around 100 nm in diameter and around 10 μ m in length. HRTEM with electron diffraction study reveals that the nanostructures are crystalline. The EDX confirms that the nanostructures contain only elements of Al and O. The XRD study shows that the nanowires are α-Al2O3. The results are explained in the light of growth mechanism based on a vapor–liquid–solid (VLS) process.

Field emission study of SiC nanowires/nanorods directly grown on SiC ceramic substrate

Single crystalline silicon carbide (SiC) nanowires were grown directly on the surface of bulk SiC ceramic substrate in a catalyst-assisted thermal heating process. The morphology of the nanowire film and the diameter of nanowires were found to be sensitive to the thickness of catalyst film and both of them had a strong effect on field emission performance. Very low turn-on and threshold fields for electron emission were observed with SiC nanowires of small diameter. A model is proposed to qualitatively explain the field emission findings, which assumes the occurrence of an insulator-to-metal-like transition in a field emitting nanowire.

Vacuum gap dependence of field electron emission properties of large area multi-walled carbon nanotube films

Field electron emission properties of aligned multi-walled carbon nanotube films were studied with variation of the vacuum gap d between anode and cathode. With d varying in the range of 0.4-2 mm, the emission current-gap voltage characteristics and the corresponding Fowler-Nordheim (FN) plots show distinct nonlinearity and regular changes with electrode separation. Three field enhancement factors may be derived from the three linear sections of a FN plot. Their variation with gap d results in different behaviours; significantly a drop of five times in the field enhancement factor is observed. The physical process responsible for our findings is suggested to be the space charge effect and both theoretical and experimental evidence is provided to support our arguments. The implication of our findings in technical applications is also discussed.

Physical origin of nonlinearity in the Fowler–Nordheim plot of field-induced emission from amorphous diamond films: Thermionic emission to field emission

Nonlinearity is observed in Fowler–Nordheim (FN) plots of field emission from nondoped and nitrogen-doped amorphous diamond films. Based on a unified electron emission equation a detailed analysis is carried out. The results from numerical calculation of the unified equation are consistent with the experimental data. It is shown that the nonlinearity in the FN plot originates from a transition from thermionic emission to field emission as the applied field increases. The electrical field ranges are derived in which the field emission and thermionic emission approximation applies.

Synthesis of silicon carbide nanowires in a catalyst-assisted process

Abstract At elevated temperatures, silicon carbide nanowires were synthesized in a catalyst-assisted process using aluminum as a catalyst. Transmission electron microscopy shows that the nanowires are around 20 nm in diameter and around 2 μm in length. High resolution transmission electron microscopy shows that the nanowires are crystalline β-SiC. Raman spectra show the typical features of nano-SiC. A model based on vapor–liquid–solid process is proposed to explain our finding.