Field emission properties of the graphenated carbon nanotube electrode

Highlights

• Facile method to prepare graphenated carbon nanotubes (g-CNTs).

• The electric field emission behaviour of g-CNTs was studied.

• g-CNTs show better emission current stability than non-graphenated CNTs.

Abstract

Reduced graphene oxide-coated carbon nanotubes (RGO-CNT) electrodes have been prepared by hot filament chemical vapour deposition system in one-step growth process. We studied RGO-CNT electrodes behaviour as cold cathode in field emission test. Our results show that RGO-CNT retain the low threshold voltage typical of CNTs, but with greatly improved emission current stability. The field emission enhancement value is significantly higher than that expected being caused by geometric effect (height divided by the radius of nanotube). This suggested that the field emission of this hybrid structure is not only from a single tip, but eventually it is from several tips with contribution of graphene nanosheets at CNT's walls. This phenomenon explains why the graphenated carbon nanotubes do not burn out as quickly as CNT does until emission ceases completely. These preliminaries results make nanocarbon materials good candidates for applications as electron sources for several devices.

Introduction

Field emission efficiency is determined by a combination of factors such as conductivity, surface work function and geometry of the emitter [1]. An ideal field emitter must be a good electrical conductor with low work function, high enhancement factor (β) and be stable at fairly high emission current density. Carbon-based materials are suitable for use as cold cathode emitters, because of the low voltages required to extract electrons from its surface. Among carbon-based materials, diamond (mainly sp³ hybridized state of carbon), carbon nanotubes (CNTs) and graphene (both sp² hybridized state of carbon) have low power consumption, potential for miniaturization, and outstanding field emission behaviour. All these properties make attractive candidates for applications as electron sources for several devices [2].

The emission of electrons from diamond in vacuum occurs readily as a result of the negative electron affinity (NEA) the hydrogenated surface due to features with nanoscale dimensions, which can concentrate electric fields high enough to induce electron emission from them [3], [4]. It has been direct measured by Chatterjee et al. [4] that the emission came from the grain boundaries (sp²-rich region) and not the protruding regions. In contrast, excellent electron emission behaviour of CNTs is related to high-aspect-ratio geometry [1]. To date, very good results from both carbon hybridizations have been reported with currents of 1 mA cm⁻² for threshold fields as low as 2 V μm⁻¹ [5].

The NEA and chemical stability of doped diamond and nanocrystalline diamond make them highly stable field emitter material [2], however with higher threshold field than high-aspect-ratio geometry emitters. The field-emission properties of diamond emitters depend upon the thickness, density, microstructure and sp³/sp² carbon ratio in the films with typical threshold field values ranging from 5 to 50 V μm⁻¹ [7], [8], [9], [10]. Lower threshold field from diamond electrodes were reported from samples with high β values, which could require complex and expensive micro-fabrication [6], [8]. High β values are usually required to pattern and etch the diamond into suitable pyramids or needle shapes, although recently diamond nanocones or microstructured have been made using CVD process and show promising field-emission characteristics [7], [9]. Xiao et al. reported low threshold field emission ∼2.5 V μm⁻¹ to nested cones of graphitically bonded carbon (graphene sheets) covered with nitrogen-doped ultrananocrystalline diamond (N-UNCD), however no current transient was showed [8].

Zou et al. [9] and Zanin et al. [7] recently showed field emission studies of microstructured diamond films deposited onto a densely packed “forest” of vertically aligned multiwalled carbon nanotubes (VACNT). On both works, field-emission tests of diamond/carbon nanotube composite show the typical low threshold voltages for carbon nanotube structures (2 V μm⁻¹) but with better stability and longer lifetime up to 35 h [9] and 75 h [7].

Getty et al. reported a field emission study of N-UNCD films grown on polished Si and contrast with N-UNCD films grown on sharpened Si tip arrays and Si ridges [2]. Authors showed the electron emission from planar N-UNCD films are fairly comparable to the emission from films grown on sharpened Si tip arrays and Si ridges, indicating that field emission in these materials is dominated by electron emission from nanoscale grain boundaries [2]. However, it was not clear why the planar N-UNCD film exhibits lower field emission threshold voltages compared to sharp emitters tested, which is unexpected. It is remarkable the life testing of electron emission from N-UNCD films, which showed the emission with low current degradation over 1000 h. Although with considerable fluctuation, emission stability of these diamond electrodes has been proved better than CNTs or graphene.

The field emission of CNTs is problematic because they burn out during emission, ceasing the emission completely. Significant research efforts have been performed to improve field emission stability of carbon nanotubes. Chen et al. [10] improved field emission stability of thin film of MWCNT using a tip sonication treatment. Authors showed field emission current of thin-MWCNT film at high current density was quite stable up to 19 h and after tip sonication treatment that stability showed to be longer ∼30 h. Field emission stability was significantly improved during a long period of operation probably because many shortened thin-MWCNTs could then participate of the emission after the treatment. Pandey et al. [11] showed strontium titanate coated carbon nanotube matrices with low emission thresholds of 0.8 V μm⁻¹ and very stable electron field emission for 40 h. Authors claimed low emission threshold was obtained due to tunnelling width as a function of the SrTiO₃ thickness.

Recently, significant research efforts have been centred on graphene due its electronic properties, which are very similar or even better than CNTs [12], [13], [14], [15]. Consequently, much effort has been made to prepare and to apply reduced graphene as a field emission material. The electric field emission from graphene is challenging especially when the sheets lay on the substrate surface, resulting in low field enhancement [16]. Nevertheless, Malesevic et al. [17] showed that graphene could be deposited vertically aligned to substrate, which increased the field enhancement factor. Quian et al. [18] showed that the field emission from graphene oxide (GO) nanosheets prepared by Hummer method have excellent and stable field emission properties with a low threshold field (∼1.5 V μm⁻¹).

Huang et al. [19] showed the field emission properties of GO are found to be a non-monotonic function of the C/O ratio in a wide range of 2.06–14.80. Samples with C/O ratio of 6.98 show the lowest turn-on (∼1.8 V μm⁻¹) and longest-time (10 h) current stability. In agreement with Huang, Kung et al. showed the emission current of vertically aligned (VA) CNT array increased ∼800% along with a decrease of the onset field emission voltage from 0.8 to 0.6 V/μm, when treated by oxygen or ozone compared to as-grown samples [20]. Mathur et al. showed that oxygen plasma treatment opens VACNTs’ tips enhancing its field emission behaviour [22]. Authors measured the turn-on electric field ∼0.80 V μm⁻¹ from untreated VACNTs and ∼0.60 V μm⁻¹ from open ended VACNTs samples. These open VACNTs have more defects sites such as edge defects from the edge planes of the graphene sheet [21]. Kurt et al. [22] reported, for the first time, carbon nanotubes decorated with graphene nanosheets growing radial to the tube axis and Parker et al. [23] proposed a model that involves the build-up of residual strain followed by buckling to nucleate a graphitic-edge protrusion from the CNTs’ sidewall.

All these literature have inspired us to prepare and study field emission behaviour of graphenated CNTs samples. Herein we demonstrate that the major advantage of these hybrid structures is their significant improvement in emission current stability due to multiple-graphene edges covering CNTs.