Yu Dian Lim

Field emission properties of SiO2-wrapped CNT field emitter

Carbon nanotubes (CNTs) exhibit unstable field emission (FE) behavior with low reliability due to uneven heights of as-grown CNTs. It has been reported that a mechanically polished SiO2-wrapped CNT field emitter gives consistent FE performance due to its uniform CNT heights. However, there are still a lack of studies on the comparison between the FE properties of freestanding and SiO2-wrapped CNTs. In this study, we have performed a comparative study on the FE properties of freestanding and SiO2-wrapped CNT field emitters. From the FE measurements, freestanding CNT field emitter requires lower applied voltage of 5.5 V μm-1 to achieve FE current density of 22 mA cm-2; whereas SiO2-wrapped field emitter requires 8.5 V μm-1 to achieve the same current density. This can be attributed to the lower CNT tip electric field of CNTs embedded in SiO2, as obtained from the electric field simulation. Nevertheless, SiO2-wrapped CNTs show higher consistency in FE current than freestanding CNTs. Under repeated FE measurement, SiO2-wrapped CNT field emitter achieves consistent FE behavior from the 1st voltage sweep, whereas freestanding field emitter only achieved consistent FE performance after 3rd voltage sweep. At the same time, SiO2-wrapped CNTs exhibit better emission stability than freestanding CNTs over 4000 s continuous emission.

Enhanced Carbon Nanotubes Growth Using Nickel/Ferrocene-Hybridized Catalyst

Tall, crystalline carbon nanotubes (CNTs) are desired to successfully integrate them in various applications. As the crystallinity of CNTs improves with increasing growth temperatures, higher growth temperatures are required to obtain crystalline CNTs. However, in a typical chemical vapor deposition (CVD) process, CNT growth rate reduces when the growth temperature exceeds a specific level due to the degradation of the catalyst particles. In this study, we have demonstrated the improved catalytic activity of nickel/ferrocene-hybridized catalyst as compared to sole ferrocene catalyst. To demonstrate this, CNTs are grown on bare silicon (Si) as well as nickel (Ni) catalyst-deposited substrates using volatile catalyst source (ferrocene/xylene) CVD at the growth temperatures ranging from 790 to 880 °C. It was found that CNTs grown on bare Si substrate experience a reduction in height at growth temperature above 860 °C, whereas the CNTs grown on 10 nm Ni catalyst-deposited substrates experience continuous increase in height as the temperature increases from 790 to 880 °C. The enhancement in the height of CNTs by the addition of Ni catalyst is also demonstrated on 5, 20, and 30 nm Ni layers. The examination of CNTs using electron microscopy and Raman spectra shows that the additional Ni catalyst source improves the CNT growth rates and crystallinity, yielding taller CNTs with a high degree of structural crystallinity.

Field emission characteristics of short CNT bundles

The outstanding field emission characteristics of Carbon Nanotubes (CNT) offer promising potential for a wide range of vacuum electronics applications. In this study, short (1 μm) CNT arrays were grown selectively on SiO2/Si substrates using ferrocene/xylene catalyst source. High field emission current density of 366.7 mA/cm2 was obtained. A common field enhancement factor (β) value of 326 was obtained for CNT bundles with various separation distances.

Enhanced field emission properties of carbon nanotube bundles confined in SiO2 pits

It has been widely reported that carbon nanotubes (CNTs) exhibit superior field emission (FE) properties due to their high aspect ratios and unique structural properties. Among the various types of CNTs, random growth CNTs exhibit promising FE properties due to their reduced inter-tube screening effect. However, growing random growth CNTs on individual catalyst islands often results in spread out CNT bundles, which reduces overall field enhancement. In this study, significant improvement in FE properties in CNT bundles is demonstrated by confining them in microfabricated SiO2 pits. Growing CNT bundles in narrow (0.5 μm diameter and 2 μm height) SiO2 pits achieves FE current density of 1-1.4 A cm-2, which is much higher than for freestanding CNT bundles (76.9 mA cm-2). From the Fowler Nordheim plots, confined CNT bundles show a higher field enhancement factor. This improvement can be attributed to the reduced bundle diameter by SiO2 pit confinement, which yields bundles with higher aspect ratios. Combining the obtained outcomes, it can be conclusively summarized that confining CNTs in SiO2 pits yields higher FE current density due to the higher field enhancement of confined CNTs.