Self-organized carbon nanotubes grown at the grain boundary of iron-nitride

Chemical vapor deposition of carbon nanotubes (CNTs) is the most promising method for fabricating CNTs based nanodevices [1] and sensors [2] because CNTs can be selectively grown by structuring the catalyst in a predefined arrangement. However, only dense aligned CNTs can be produced using transition metal catalysts (Fe, Co, and Ni) by plasma-enhanced chemical vapor deposition (PECVD) [3]. It is well known that electric field shielding effects from closely packed arrays of nanotubes adversely affect their field emission characteristics [3,4]. Therefore, it is desirable to control the density of CNTs so as to reduce the electric field shielding effect from adjacent CNTs. Although the density of CNTs can be controlled by various patterning technologies [3–5], it is still crucial to develop self-organizing techniques that do not require any pre-patterning for the growth of CNT arrays. In addition, it has been found that the nucleation and growth of CNTs may be significantly enhanced by introducing N2 or NH3 in the CVD process [6]. Although the mechanism of the enhanced CNT growth in N2/NH3 environment is not yet clear, nitriding the catalyst surface seems to play an important role in CNT growth [7,8]. In this work, we show that aligned CNTs are preferably grown at the grain boundaries of iron nitride (Fe–N) films. This simple growth technique may lead to the development of controllable self-assembled CNT arrays for electron emitter and sensor applications. Fe–N films were deposited on n-type Si wafers by the filtered cathodic vacuum arc (FCVA) technique [9]. An iron target of 99.99% purity was used. The deposition pressure was kept at 5 · 10 Torr by adjusting the nitrogen (purity 99.99%) gas flow rate. The arc current was set to 100A. The nitrogen content of the Fe–N film was determined to be around 20 at.% by X-ray photoelectron spectroscopy (XPS; VG Micro Lab 310-F) using Mg Ka radiation at 1253.6eV. The Fe–N film was then placed into the centre of a tube furnace. The base pressure of the reactor was evacuated to 10 Torr using a rotary pump. It took about 10min to heat up the substrate from room temperature to 600 C with the flowing gas mixture of H2 and N2 in a ratio of 90– 30sccm. Acetylene (C2H2) of 10sccm was then introduced into the chamber when the temperature reached 600 C. The growth pressure was maintained at 10– 20Torr during the CVD process and the growth time was set at 5min. In addition, a negative dc bias could also be applied to the substrate which promotes the growth of aligned CNTs. Fig. 1 shows scanning electron microscope (SEM) images of the CNT growth from the Fe–N catalyst. Surprisingly, the CNTs are preferably grown at the grain boundaries of the Fe–N film (Fig. 1(a)). High-magnification SEM images (Fig. 1(b)) indicate that the Fe–N grains are surrounded by vertically aligned CNTs when the sample was prepared under zero bias condition. When a negative dc bias of 300V was applied to the substrate during CNT growth, relatively longer, straighter and well aligned CNTs were obtained (Fig. 1(c)). It is shown that CNTs are selectively grown from the FeN layer with a regular pattern which is similar to the CNT array growth from patterned catalysts [3]. The microstructures of CNTs grown by thermal CVD with and without bias were characterized by transmission electron microscopy (TEM) at 200kV. The typical length and diameter of the CNTs are around 0.5– 1.5lm and 30–150nm respectively. Fig. 2(a) shows an