Enhanced field emission from density-controlled SiC nanowires

doi:10.1016/j.matchemphys.2008.05.024

Materials Chemistry and Physics

Volume 112, Issue 1, 15 November 2008, Pages 88-93

https://doi.org/10.1016/j.matchemphys.2008.05.024 Get rights and content

Abstract

Silicon carbide (SiC) nanowires were grown directly on Si substrates by thermal evaporation of WO₃ and graphite powders at high temperature using NiO catalyst. The densities of the nanowires were controlled by varying the NiO catalyst concentration. The morphology, structure and composition of the nanowires were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman, FTIR, transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) measurements. The synthesized nanowires were single crystalline β-SiC oriented along the [1 1 1] direction. Based on the experimental results, a possible growth mechanism was explained on the basis of solid–liquid–solid (SLS) growth model. Field emission measurements showed that the emission efficiency was strongly dependent on the density of SiC nanowires. Lowest turn-on field of 1.8 V μm⁻¹ and highest field enhancement factor of 5.9 × 10³ was observed for the medium density SiC nanowire sample.

Introduction

One-dimensional (1D) nanomaterials (nanowires, nanorods or nanotubes) are attracting increasing interest due to their remarkable optical, electrical, and physical properties and their potential applications in nano-electronic devices [1], [2]. Field emission is one of the most fascinating properties of one-dimensional nanostructured materials and has been extensively studied due to its importance both in view of fundamental science and in high-tech applications. A great deal of intensive research interests are driven by the enormous commercial applications of the vacuum electronic devices using nanostructures as cathode materials, such as field emission flat displays [3], X-ray sources [4], and microwave devices [5]. For many practical applications, field emission from a cold-cathode is required to have low turn-on and threshold fields with uniform and high emission current. It has been reported experimentally and theoretically that the density of nanostructures plays a crucial role for better field emission properties [6], [7], [8], [9], [10]. A decrease (low density) or increase (high density) in the density of nanostructures has resulted in degraded emission properties. Thus, the density and morphology of the nanostructures must be better controlled for enhanced field emission performance.

Various types of one-dimensional nanostructures, such as carbon nanotubes, semiconductor nanowires (both wide and narrow band-gap materials) have been studied extensively for their field emission properties [6], [11], [12], [13]. Wide band-gap semiconductors are considered to be promising for field emission applications because of their low electron affinity, chemical stability and low work function, which is advantageous as Fowler–Nordheim tunneling current is very sensitive to the work function. β-Silicon carbide (SiC) nanowires with a wide band-gap of 2.3 eV have shown excellent field emission properties due to their superior electronic, physical and chemical properties [13], [14], [15], [16], [17]. Various methods have been employed to synthesize SiC nanostructures including thermal evaporation, chemical vapor deposition and carbothermal reduction [13], [14], [15], [16], [17], [18], [19], [20], [21]. Most of these reported synthesis methods required a vacuum environment and silicon/carbon source materials to grow SiC nanowires. In this study, we have used a simple direct synthesis method [22] to grow SiC nanowires by heating the NiO catalyzed silicon substrate in argon atmosphere without using any additional silicon source materials. The density of nanostructures plays an important role for the enhanced field emission characteristics and also there are only few reports available on the field emission characteristics of density and size-controlled SiC nanostructures [23], [24]. Earlier, we have reported a turn-on field of 4.2 V μm⁻¹ for the high density SiC nanowires [22]. Here we controlled the density of SiC nanowires by changing the concentration of the NiO catalyst and demonstrated that the field emission characteristics could be improved by tuning the density of SiC nanowires. A solid–liquid–solid (SLS) growth mechanism was proposed to explain the formation of SiC nanowires.

Section snippets

Synthesis of SiC nanowires

Core-shell SiC–SiO₂ nanowires were grown directly on Si substrates by carbothermal reaction of tungsten oxide (WO₃) with graphite (C) using NiO catalyst. The growth experiments were carried out in a conventional two-zone horizontal tube furnace (Fig. 1). The furnace quartz tube is 55 cm long and each zone has a uniform-temperature region of 10 cm long. The n-type Si(1 0 0) wafers (0.005 Ω-cm) of size 2 cm × 2 cm were used as the substrates as well as the Si source material where as carbothermal

SEM analysis

The surface morphology of the SiC nanowires grown at different NiO catalyst concentrations was analyzed by SEM. Fig. 2a–c shows SEM images of the SiC nanowires grown at different NiO catalyst concentrations. SEM images showed straight, curved, randomly oriented, and freestanding SiC nanowires. The uniformity of the grown nanowires was confirmed by obtaining SEM images at different areas of the sample. The SEM images clearly showed that SiC nanowires with different densities (low density, medium

Conclusions

In summary, we synthesized single crystalline β-SiC nanowires on NiO catalyzed Si substrates using the carbothermal reaction of WO₃ and graphite powders at high temperature. We have shown that the density of SiC nanowires could be varied by changing the concentration of the NiO catalyst. A solid–liquid–solid (SLS) growth mechanism was proposed for the growth of SiC nanowires. The field emission measurements showed that the field emission properties of the SiC nanowires could be enhanced by

Acknowledgements

This work was supported by the Korean Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2005-005-J13101), and grant no. RTI04-01-04 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry and Energy (MOCIE).

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Enhanced field emission from density-controlled SiC nanowires

Abstract

Introduction

Section snippets

Synthesis of SiC nanowires

SEM analysis

Conclusions

Acknowledgements

Appl. Surf. Sci.

Chem. Phys. Lett.

J. Cryst. Growth

Chem. Phys. Lett.

Int. J. Ref. Metals Hard Mater.

Carbon

Appl. Phys. Lett.

Adv. Mater.

Science

Appl. Phys. Lett.

Nature

Adv. Mater.

Appl. Phys. Lett.

Appl. Phys. Lett.

Adv. Mater.

Jpn. J. Appl. Phys.