Oxidation Studies of Carbon Nanotubes for Applications as X-Ray Field Emitters Using an Aberration-Corrected, Environmental TEM.
Oxidation Studies of Carbon Nanotubes for Applications as X-Ray Field Emitters Using an Aberration-Corrected, Environmental TEM.
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使用像差校正环境 TEM 对用作 X 射线场发射器的碳纳米管进行氧化研究。
DOI：
10.1017/s1431927613004327
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
Sinclair,Robert
中科院分区：
文献类型：
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作者：
Koh,AiLeen;Gidcumb,Emily;Zhou,Otto;Sinclair,Robert
自1991年碳纳米管（CNT）[1]被发现以来，其应用越来越多，最值得注意的是作为X射线管中的场发射电子源用于医疗应用[2，3]。在实验室环境中，碳纳米管的场发射测量通常是在超高真空系统中进行的，基础压力为~ 10-7 mbar或更好。在不太严格的真空条件下，发现CNT表现出较低的发射电流和缩短的寿命[4，5]。在发现CNT后不久，几个小组试图利用氧化过程来操纵它们的结构，例如通过打开它们的终止帽或通过使管变薄[6，7]。在文献中，这些氧化步骤通常在外部实验室环境中进行，并且用透射电子显微镜（TEM）事后调查氧化样品的状态。然而，由于其纳米尺寸，尚未对其氧化的潜在机制进行直接研究。在本文中，我们报告了使用像差校正环境TEM（ETEM）原位氧化碳纳米管时对其结构变化的直接研究。首先将样品在高真空中加热到300 ℃，并确定了一些纳米管用于跟踪。然后，在电子束消隐的情况下，将1.5毫巴的研究级（99.9999%纯度）氧气引入ETEM中15分钟，同时将温度保持在300 ℃。在该循环结束时，将气体从ETEM中清除，同时将温度保持在300 ℃，并对相同的纳米管进行成像以识别暴露于氧气后的任何差异。然后将温度升高至400 ℃，重复氧化过程，并在从系统中清除氧气后，在400 ℃下对同一组纳米管进行跟踪和成像。这些氧化过程在安装在不同TEM网格上的样品上重复进行，起始温度和结束温度分别为400 ℃和520 ℃
Since their discovery in 1991 carbon nanotubes (CNTs)[1] have found an increasing number of applications, most notably as field emission electron sources in X-ray tubes for medical applications [2, 3]. In a laboratory setting, field emission measurements of CNTs are usually carried out in an ultrahighvacuum system with base pressure of~ 10-7 mbar or better. Under less stringent vacuum conditions, CNTs are found to exhibit lower emission currents and reduced lifetimes [4, 5]. Shortly after the discovery of CNTs, several groups attempted to utilize the oxidation process to manipulate their structures, for instance by opening up their terminating cap or by thinning the tubes [6, 7]. In the literature, these oxidation steps were usually performed in an external laboratory setting, and the state of the oxidized samples was surveyed a posteriori with a transmission electron microscope (TEM). However, because of their nanoscale, no direct study has been performed on the underlying mechanism of their oxidation.In this paper, we report the direct study on the structural changes in CNTs as we oxidize them in-situ using an aberration-corrected environmental TEM (ETEM). The samples were first heated to 300C in high-vacuum and a few nanotubes were identified for tracking. Then, with the electron beam blanked, 1.5 mbar of research grade (99.9999% purity) oxygen was introduced into the ETEM for 15 min while maintaining the temperature at 300C. At the end of this cycle, the gas was purged from the ETEM while the temperature was kept at 300C, and the same nanotubes were imaged to identify any differences after having been exposed to oxygen. The temperature was then increased to 400C, the oxidation process was repeated, and the same set of nanotubes was tracked and imaged at 400 C after oxygen was purged from the system. These oxidation procedures were repeated on samples mounted on different TEM grids with start and end temperatures of 400C and 520C, respectively