Atomic Layer Deposition of Buffer Layers for the Growth of Vertically Aligned Carbon Nanotube Arrays
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Vertically aligned carbon nanotube arrays (VACNTs) show a great potential for various applications, such as thermal interface materials (TIMs). Besides the thermally oxidized SiO2, atomic layer deposition (ALD) was also used to synthesize oxide buffer layers before the deposition of the catalyst, such as Al2O3, TiO2, and ZnO. The growth of VACNTs was found to be largely dependent on different oxide buffer layers, which generally prevented the diffusion of the catalyst into the substrate. Among them, the thickest and densest VACNTs could be achieved on Al2O3, and carbon nanotubes were mostly triple-walled. Besides, the deposition temperature was critical to the growth of VACNTs on Al2O3, and their growth rate obviously reduced above 650 °C, which might be related to the Ostwald ripening of the catalyst nanoparticles or subsurface diffusion of the catalyst. Furthermore, the VACNTs/graphene composite film was prepared as the thermal interface material. The VACNTs and graphene were proved to be the effective vertical and transverse heat transfer pathways in it, respectively.
KeywordsAtomic layer deposition Vertically aligned carbon nanotube arrays Oxide buffer layers Thermal interface materials
Atomic layer deposition
Chemical vapor deposition
Differential scanning calorimeter
Field emission scanning electron microscopy
Laser flash thermal analyzer
Radial breathing modes
Transmission electron microscopy
Thermal interface materials
Vertically aligned carbon nanotubes
Vertically aligned carbon nanotube arrays (VACNTs) have various outstanding performances and show great potential for a wide variety of applications. Due to their high axial thermal conductivity, many VACNT-based thermal interface materials (TIMs) have been developed for thermal packaging applications [1, 2, 3, 4, 5, 6, 7]. To synthesize the high-quality VACNTs on different substrates, chemical vapor deposition (CVD) has been commonly used, and the buffer layer should be deposited on the substrate before the deposition of the catalyst, such as Fe. Generally, the buffer layers are used to prevent the diffusion of the catalyst into substrates, so it is also very important to achieve the high-quality buffer layers on different substrates.
Atomic layer deposition (ALD) has self-limited behavior, which could achieve pinhole-free, dense, and conformal films on complex non-planar substrates . Recently, many researchers have used it to deposit the buffer layers for the growth of VACNTs [9, 10, 11]. Amama et al. reported the water-assisted CVD of VACNTs using ALD Al as the buffer layer . Quinton et al. reported the floating catalyst CVD of VACNTs using Fe as the catalyst. They found that VACNTs had faster nucleation rate and more uniform tube diameter on ALD Al2O3 buffer layer, compared with SiO2 . Compared with thermal and microwave plasma SiO2, the VACNTs grown on ALD SiO2 had the fastest nucleation rate . Yang et al. reported that VACNTs could be synthesized on non-planar substrates using ALD Al2O3 as the buffer layer and Fe2O3 as the catalyst, respectively . Compared with the planar surface, the non-planar surface could largely increase the specific surface area, which would be very beneficial for the preparation and further applications of VACNTs [12, 13, 14]. Although some ALD oxide buffer layers have been synthesized for the growth of VACNTs, their role was still not very clear in the CVD process.
In this research, we used CVD to prepare the VACNTs with different buffer layers, including ALD Al2O3, ALD TiO2, ALD ZnO, and thermally oxidized SiO2. The effects of different oxide layers and deposition temperature on the growth of VACNTs were analyzed. Besides, the VACNTs/graphene composite film was also developed as the thermal interface material, and the VACNTs were used as the additional vertical thermal transfer pathways in it.
Al2O3, ZnO, and TiO2 thin films were deposited on Si substrates by ALD, and SiO2 was formed on Si substrate by thermal oxidization. Trimethylaluminum (TMA), tetrakis(dimethylamino)titanium (TDMAT), and diethylzinc (DEZ) were used as the precursors for ALD of Al2O3, TiO2, and ZnO films, respectively. For all of them, H2O was used as the oxygen source, and the deposition temperature was set at 200 °C. The thickness of Al2O3, ZnO, and TiO2, and SiO2 films was 20 nm. One-nanometer-thick Fe film was deposited on all of them by electron-beam (EB) evaporation, where it was used as the catalyst. The CVD method was applied to synthesize the VACNTs based on a commercial CVD system (AIXRON Black Magic II). Before the growth of VACNTs, the catalyst was annealed in the hydrogen (H2) atmosphere at 600 °C. The period was 3 min, and the flow rate of H2 was set at 700 sccm. After that, the acetylene (C2H2) and H2 were introduced into the chamber, and then VACNTs were prepared. The flow rates of C2H2 and H2 were 100 and 700 sccm, respectively. The deposition temperature was changed from 550 to 700 °C, and the period was fixed at 30 min.
where λ and ρ were the thermal conductivity and density of the composite film, respectively.
Results and Discussion
The growth of VACNTs has been analyzed on different oxide buffer layers, such as ALD Al2O3, ALD TiO2, ALD ZnO, and thermally oxidized SiO2. Among them, VACNTs were the thickest and densest on Al2O3, which indicated that the lifetime of catalyst nanoparticles was the longest and the vertical alignment of VACNTs was the best on it. Besides, the VACNTs were found to be multilayer on Al2O3, and the deposition temperature was very critical to the growth of VACNTs. Compared with SiO2, the nucleation and initial growth of VACNTs were more easily achieved on Al2O3, which resulted in a higher density of VACNTs on it. After the growth of VACNTs on Al2O3, they were used to prepare the composite film together with graphene and epoxy resin. Compared with the pure epoxy resin, the vertical and transverse thermal conductivities of the composite film have been largely improved.
The authors thank Dr. Yong Zhang from the School of Automation and Mechanical Engineering, Shanghai University, for the useful discussions.
This work was financially supported by the National Natural Science Foundation of China (Nos. 61704102 and 51861135105).
Availability of Data and Materials
The datasets supporting the conclusions of this article are included with the article.
HHL, GJY, BS, XXZ, and HPM designed the experiments and analyzed the data. HHL, GJY, BS, XXZ, HPM, YZT, HLL, and JL discussed the results and contributed to the writing of the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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