Since its first discovery by Iijima in 1991 [1], carbon nanotubes (CNTs) have attracted tremendous scientific and technological interest because of their exceptional structural, electrical and mechanical characteristics [2] for many potential applications, including that the high-strength composites [3], energy storage [4], nanofabrication [5], etc. In particular, the field emission has been considered as one of the most promising characteristics of CNTs to be realized for immediate applications owing to the favorable features for field emitters, such as high aspect ratio, high electrical conductivity, high mechanical strength, high chemical stability, as well as high thermal conductivity inherent to CNTs [6][7][8].
Indeed, excellent field emission properties, namely high emission current density, low emission threshold voltage, and long lifetime have been demonstrated for emitters made of CNTs [7]. Nevertheless, in order to optimizing the field emission properties for large area applications, it is highly desirable to produce vertically aligned CNTs with optimal combinations of density, diameter and length on the substrates. The field emission properties of CNTs are often affected by these factors, especially in the device applications [9,10]. Consequently, synthesizing CNTs on catalytic transition metal patterned substrates by chemical vapor deposition (CVD) systems have been widely used for obtaining better control of those parameters [8]. Unfortunately, contamination has been known to occur on the surface of CNTs, which may affect the stability of the emission process and reduces their efficiency. In order to enhance the field emission properties of CNTs, some effective methods are required to investigate the fundamental issues involved in optimizing the performance of CNTs. For instance, plasma surface treatment on the surface of carbon-based materials has been proposed to reduce the contamination [11][12][13] and has shown significant improvements in field emission characteristics [13]. However, there is still plenty of room for further improvements. In particular, to our knowledge, little work has been systematically carried out on how purifying