Large Current Emission from CNTs Cold Cathode

摘要

Carbon nanotubes (CNTs) have attracted great interest in the scientific field because of their unique structure, remarkable properties and potential applications. CNTs have been synthesized by methods such as arc discharge [1], chemical vapor deposition [2], laser ablation[3] etc. Such sustained, high-current field emission is necessary for many technological applications: for example, flat-panel field-emission displays require 10mA/cm{sup}2, while microwave power amplifier tubes require at least 400mA/cm{sup}2 [1]. These high-current density levels were previously obtainable only by thermionic emission above 1000K, or lithographically fabricated tips. Field emission has been reported from other forms of carbon, however, at lower current densities (0.3mA/cm{sup}2 for graphite [2], 30mA/cm{sup}2 for diamond (Refs. [3] and [4], and references therein)). CNTs cold cathode with large current density can take place of themionic cathode in power devices such as microwave amplifier tubes. In this paper, pristine Si (100) wafers were used as the substrates. We grew carbon nanobutes with thermal chemical vapor deposition (CVD). Prior to the growth, the substrates were supersonically cleaned in acetone, alcohol, and deionized water baths and subsequently, mounted into a chamber pumped to 10{sup}(-5) Torr. Then cuprum (Cu) film was vaporized onto the substrates. To study annealing effects, Si substrates coated with Cu films were annealed at a temperature range from 100～200°C for 2 hours in vacuum condition. Before patterned nickel layer (2 mm diameters) was sputtered as catalyst, Ar{sup}+ bombardment on Ni target has been performed for a few minutes to remove the oxidized surface layer on the Ni target. The specimens were placed into the quartz reactor (quartz tube of 80 cm in length and 5 cm in OD) for growing CNTs and were pumped down to less than ～10{sup}(-3) Torr using a mechanical pump. The catalyst layer was first reduced at 500°C in 50 Torr of flowing H{sub}2 (99.999% in purity) for 1 h to obtain the needed catalyst particles and enhance the catalytic activity. The substrates were then heated to a synthesis temperature while introducing H{sub}2 with a flow rate of 70 and 80 sccm, respectively. The synthesis temperature and pressure were approximately 700°C and 50 Torr, respectively. When temperature and pressure were stabilized, the hydrogen was replaced with mixture of C{sub}2H{sub}2 (99.6% in purity) and N{sub}2. The growth time was 1 h. The samples were then cooled in H{sub}2 flow ambient. After the growth, the morphologies of CNTs were examined by Scanning Electron Microscope (SEM). The morphologies of CNTs surface observed under a Scanning Electron Microscopy is shown in Fig 1. It can be seen that CNTs randomly distributed on catalyst layer and took on relatively uniform diameter about of 20～30 nm, the density of CNTs film was uniform in favor of field emission.