Preparation and characterization of carbon nanotubes encapsulated GaN nanowires

Abstract

A novel two-step catalytic reaction is developed to synthesize gallium nitride nanowires encapsulated inside carbon nanotubes (GaN@CNT). The nanowires are prepared from the reaction of gallium metal and ammonium using metals or metal alloys as a catalyst. After the formation of the nanowires, carbon nanotubes are subsequently grown along the nanowires by chemical vapor deposition of methane. The structural and optical properties of pure GaN nanowires and GaN@CNT are characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy and Raman spectroscopy. The results show that GaN nanowires are indeed encapsulated inside carbon nanotubes. The field emission studies show that the turn-on field of GaN@CNT is higher than that of carbon nanotubes, but substantially lower than that of pure GaN nanowires. This work provides a wide route toward the preparation and applications of new one-dimensional semiconductor nanostructures.

Introduction

Since the discovery of carbon nanotubes by Iijima [1], the materials always offer intriguing possibilities for fundamental studies of nanoscale well-defined structures and provide high potential for technological applications. Significant progresses have been made recently on the studies of mechanical, electronic transport and structural properties of individual carbon nanotubes and the results indicated that they exhibited high tensile strength [2], [3], discrete electronic states [4] and structural helicity [5]. On the application side, carbon nanotubes have been applied in the fabrications of various devices, for examples, scanning probes [6], biological sensors [7], [8], electronic transistors [9], [10], field emitting devices [11], [12], and energy storage [13].

Filling materials into the inner hollow cavity of carbon nanotubes have brought great attention because the new filled one-dimensional (1-D) structures are expected to exhibit different physical properties than those of empty nanotubes [14]. Many attempts have been made to encapsulate various materials into carbon nanotubes. For example, capillary force has been utilized to fill lead and bismuth into an open nanotube [15], [16]. Also, different transition metals such yttrium, manganese, iron or gadolinium have been encapsulated into nanotubes using an arc-discharge method [17], [18]. In addition, a direction chemical vapor deposition (CVD) of metal organic complexes of Fe(CO)₅ and Co(CO)₅ has been used to generate the carbon nanotubes filled with Fe and Co nanoparticles [19], [20]. However, in those methods, the overall yield of the encapsulated nano-materials is rather low and they are difficult to scale up. Moreover, to our knowledge, still there are few reports on how to fill binary semiconductor materials into carbon nanotubes, although it has been known that 1-D semiconductor nanostructures (nanowires) are electronic confined systems ideal for fundamental studies of their physical properties and for the fabrications of optoelectronic nanodevices [21], [22].

One of the III–V 1-D semiconductor nanostructures, GaN nanowires, has attracted much interest because of its great potential for new visible and UV optoelectronic applications [23], [24]. Recently, we have developed a large-scale synthesis of GaN nanowires from the catalytic reaction of gallium and ammonium using various metals as a catalyst [25]. Subsequently, breakthroughs have been made to achieve high purity and high quality synthesis of the GaN nanowires by careful optimizing the growth conditions [26]. From our previous results, we found that the resulting GaN nanowires from the catalytic reaction are usually terminated with a nanoparticle on the tip of each individual nanowires and the mechanism of the nanowire formation can be attributed to vapor–liquid–solid (VLS) growth. It has been well known that single-wall and multi-wall carbon nanotubes can be prepared by a CVD of methane or ethylene using iron, nickel, cobalt or their alloys as a catalyst [27], [28], [29]. There are several similarities between the proposed mechanisms and growth conditions of GaN nanowires and carbon nanotubes. First, the growths of the nanowires and nanotubes are both performed in a gas phase with the presence of metal catalysts. Second, the catalysts of cobalt, nickel and iron can commonly serve in either one of the growths. Third, after the growth, catalytic nanoparticles were generally found on the tip of individual nanowires and nanotubes. Thus, on the basis of the similarities between their growth conditions, we propose a two-step catalytic reaction (Fig. 1) in a gas phase for the growth of GaN nanowires encapsulated inside carbon nanotubes (GaN@CNT). The resulting materials of GaN@CNT were characterized using electron microscopy and Raman spectroscopy. In addition, their filed emission properties were measured to demonstrate their potential in an electronic device application.