A new approach for orientation-controlled growth of CNTs: an in-depth analysis on the role of oxygen plasma treatment to catalyst
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In this paper, a novel and easy technique is proposed for orientation controlled growth of MWNTs. The results indicate that when CNT growth was carried over the substrate, not treated with plasma, horizontal network of MWNTs was formed. Plasma treatment to the substrate prior to CNTs growth led to formation of vertically aligned MWNTs. An in situ growth as a function of plasma treatment time reveals the mechanism behind this flip process. All experiments were performed under atmospheric pressure. At every step during time-dependent growth process, CNTs were characterized using FESEM, HRTEM, and Raman spectroscopy. Iron sputtered silicon substrate was also investigated to validate the excellent formation of as-grown vertical CNTs and also to analyze the role of oxygen plasma behind the orientation-controlled growth. The as-grown CNTs over the iron sputtered silicon substrate with or without plasma treatment were characterized by FESEM and AFM. The successful orientation-controlled growth of CNTs was achieved.
KeywordsVertically aligned MWNTs Plasma treatment Growth mechanism
Invention of CNTs triggers a new era of science and worldwide, researchers have started working on development of new nanomaterials and application based upon invented nanomaterials (Sharma et al. 2016; Singh et al. 2017; Sharma and Gupta 2016). Among all nanomaterials, due to high carrier mobility, good stability, and tunable band-gap structure with small diameter, CNTs become a perfect structure for making channel for high-performance field effect transistors (FETs), integrated circuits, and other electronic applications (Zhang et al. 2016) and hence, research communities have started focusing on various application aspects of CNTs. The requirement of structural quality of CNTs is different for different types of applications. The researchers can tune the structural quality of CNTs as per the requirement, either by synthesizing the CNTs in bulk and later tuning morphology of CNTs or by tuning the CNT parameters at the time of synthesis. To eliminate post-synthesis manipulation or assembly to make CNTs useful for application purposes, we have already discussed in detail in previous papers about controlling various parameters like morphology, structure, and dimensions of CNTs during growth process (Tripathi et al. 2014a, b, c, 2015a, b, c). However, analysis of an important parameter, i.e. orientation of CNTs is still required to achieve perfect structure of CNTs. It is important due to requirement of different types of orientation for different applications. For example, horizontally aligned network of CNTs is suitable for fabricating gas sensor (Guerin et al. 2012; Lim et al. 2012; Poche et al. 2013), while vertically aligned network of CNTs is suitable for field emission (Cui et al. 2015; Lin et al. 2015). In order to fulfil this requirement, the researchers control the orientation of CNTs by applying magnetic and electric field (Lee et al. 2003; Ko and Tsukruk 2006) on substrate during growth process of CNTs. Su et al. also carried out research on the orientation of CNTs and controlled it using crystalline surface (Su et al. 2000) as well as performing CNT growth process on porous silicon substrate (Veca et al. 2012). But, so far, reported techniques for controlling the orientation of grown CNTs are very tough and complex and sometimes also damage the CNT structure (Kim et al. 2005).
In the present paper, we report a relatively simple technique to control the orientation of as-grown CNTs. In this technique catalyst-sputtered substrate was treated with oxygen plasma. The substrate as treated with oxygen plasma was then loaded to CVD and usual CNT growth process was run. As a result, vertically oriented CNTs were achieved. On the other hand, horizontally oriented CNTs were formed on the substrate which was not treated by oxygen plasma. A detailed discussion on The role of oxygen plasma treatment on iron thin film and its outcomes on CNT growth are discussed in detail in the next sections.
A thin layer of 2 nm iron catalyst was deposited on 5 × 5 mm P-type silicon (100) substrate by RF-sputtering technique. The sputtered sample was cleaned by ultrasonicator in IPA for 5 min. The process flow of CNTs growth involved Placement of cleaned substrate in CVD system, and growth was carried out at temperature ramp of 500 °C/h under environment of 60 sccm Ar as carrier gas. When temperature of CVD chamber reached 800 °C, C2H2 gas was flown into the chamber with a flow rate of 20 sccm. After 10 min, C2H2 gas flow was stopped and the heater of CVD system was switched off; and the chamber of CVD system was allowed to cool down to room temperature in the presence of Argon. After cooling, the sample was taken out. To study the effect of oxygen plasma treatment, the catalyst-coated substrate was treated with oxygen plasma treatment for 10 min. The plasma-treated substrate was then loaded into the chamber of CVD system and CNT growth process was carried out in the same way as discussed before.
The surface morphology of as-grown samples was characterized by FESEM (NOVA NANO SEM 450, FEI) and the structural quality of CNTs was studied with Micro-Raman spectrometer with 488 nm Ar+ laser (LabRAM HR800, JY) and HRTEM (Technai, 300 kV, F30, G2Stwin). The effect of oxygen plasma of the structure of catalyst was also studied using FESEM and AFM.
Results and discussion
It is concluded that oxygen plasma treatment, an easy prior growth step, can change the orientation of CNTs. CNT growth process without plasma treatment results in horizontal network and the same growth process without prior oxygen plasma treatment results in dense carpet of vertically aligned CNTs. The growth mechanism behind the attracting results was explained. In summary, the orientation as well as yield of CNTs was successfully tuned by using simpler techniques.
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