Multiwall boron carbonitride/carbon nanotube junction and its rectification behavior.

In recent years, the ternary boron carbonitride (BCN) nanotubes have attracted increasing interests because of their unique electronic properties and potential technological applications. A prime advantage of the BCN nanotubes over their carbon counterparts is the relative simplicity in manipulating the electronic structures. Theoretical calculations have predicted that the band gap of BCN nanotubes can be tailored over a wide range by chemical composition rather than by geometrical structure.1-3 The BCN nanotubes could be quite promising in applications for nanoscale electronic and photonic devices. Many efforts have been devoted to the synthesis of BCN nanotubes since the first report in 1994.4 Nanotubes with a homogeneous BCN composition or separated phases of BN and C layers have been prepared by various means of arc discharge, laser ablation, and chemical vapor deposition (CVD).5-8 The electrical transport measurements performed on BCN nanotube bundles9 and photoluminescence from large-scale BCN nanotube arrays10 have presented its semiconductor nature. Recently, a band gap of ∼2.8 eV11 for the B0.45C0.1N0.45 nanotube and a band gap of 3.89 eV12 for the BCN nanotube with ∼1:1:1 stoichiometry have been identified by X-ray photoelectron spectroscopy and cathodoluminescence, respectively. In addition, the BCN nanojunctions have been studied through varying the local chemical composition of the nanotube.1a,13 The “tunable” composition of BCN nanotube may provide us with a reliable and economical way to achieve nanotube heterojunctions for rectifying diodes, light emitting diodes, transistors, and so forth. In this Communication, the direct synthesis of massive BCN/C nanotube junctions has been realized via a bias-assisted hot-filament CVD method. The electrical transport measurements of individual nanotube junctions were carried out on a conductive atomic force microscopy (AFM). It is found that the BCN/C nanotube junctions show a typical rectifying diode behavior. The BCN/C nanotubes were grown in a hot-filament CVD apparatus, by which the single-wall BCN nanotubes have newly been synthesized.14 In this study, clean nickel wafers were used as the substrates and a dc power supply was supplied to generate glow discharge plasma between the substrate and the tantalum anode installed above a carbonized tungsten filament. The BCN/C nanotube junctions were synthesized in a continuous CVD process with a two-step growth. First, the flow rates of N2, H2, and CH4 were kept at 75, 5, and 20 sccm, respectively. When the filament was heated to about 1600 °C, a bias of 600 V was applied to produce the glow discharge plasma, and the growth started at 2.0 kPa. After growth for 10 min, the temperature of the filament was rapidly decreased to 1500 °C, and the B2H6 gas with a concentration of 5 vol % was introduced into the chamber. In this way, multiwall BCN nanotubes were grown on the top of the carbon nanotubes, and the BCN/C nanotube junctions with sharp interface were formed. Figure 1a is a typical scanning electron microscopy (SEM) image of the large-scale aligned BCN/C nanotube arrays grown on nickel substrates. The nanotubes are 8-10 μm in length and 50-150 nm in diameter. A transmission electron microscopy (TEM) image of the as-grown BCN/C nanotubes is shown in Figure 1b, which reveals that most of nanotubes consist of two sections, marked by arrows. From an enlarged TEM image (Figure 1c), the nanotube is composed of two thoroughly different parts, one side is a herringbone-like BCN nanotube while the other is a cylinder-like multiwall carbon nanotube, marked by 1 and 2, respectively. The two parts with different atomic and electronic structures make contact with each other and form a seamless nanotube, seen from high-resolution TEM image, as indicated in Figure 1d (also see Supporting Information, Figure S1). Thus, the BCN/C nanotube junctions are obtained by the sharp interface between the BCN nanotube and the carbon nanotube. Figure 1e shows the electron energy-loss spectroscopy (EELS) spectra taken from the two parts around the junction in Figure 1c, corresponding with the section 1 † Institute of Physics, Chinese Academy of Sciences. ‡ Wuhan University. § Wuhan University of Technology. Figure 1. (a) SEM image of the as-grown BCN/C nanotube junction array; scale bar ) 5 μm. (b) TEM image of BCN/C nanotube junctions; scale bar ) 200 nm. (c) A single BCN/C nanotube junction; scale bar ) 50 nm. (d) High-resolution TEM image from the area marked by rectangle in Figure 1c; scale bar ) 5 nm. (e) EELS spectra taken from the section 1 and 2 in panel c, respectively. Published on Web 07/18/2007