|
Volume 5 Issue 2 - July 4, 2008  |
|
|
Growth of CNTs on Fe-Si catalyst prepared on Si and Al coated Si substrate F.-Y. Teng, Jyh-Ming Ting*, Sahendra P. Sharma, Kun-Hou Liao
Department of Materials Science and Engineering, College of Engineering, National Cheng Kung University *Corresponding Author. Email address: jting@mail.ncku.edu.tw Nanotechnology 19, 095607 (6pp) Issue 9 (2008) |
||||
  | ||||
Carbon nanotubes (CNTs) continue
to draw tremendous attentions from the entire research community
since its discovery in 1991 [1]. These fascinating properties
of CNTs depend on their structures, which are strongly influenced by
the synthesis methods and parameters. Among the parameters,
characteristics of the catalyst play a key role in controlling the
structure of CNTs at molecular or nano scale. Typical
catalysts that are widely employed in CNT synthesis include
transition metals such as Co, Fe, Ni, Mo, and their combination such
as Fe/Ni, Co/Ni, Y/Ni [2-4]. It is also known that when the
formation of silicide between a catalyst and Si substrate, e.g.,
iron silicide, is highly undesirable. Various types of
thin film materials have therefore used as an interlayer to prevent
silicide formation [5-8]. In this paper, the use of a thin Al
layer, ranging from 2 nm to 12 nm, between a 24-nm thick Fe-Si
catalyst and a silicon substrate is reported. We show
that the Al interlay not only prevents the silicide formation but
also greatly enhanced the CNT growth rate at growth temperature of
only 370 ℃ in a microwave plasma enhanced chemical vapor deposition
(MPCVD) reactor. Furthermore, the use of such an Al interlayer
is so effective as normally required catalyst etching prior to the
growth is waived.
Fig. 1 shows a TEM cross sectional image of an etched Fe-Si catalyst which is bounded by the two dotted lines. It is seen that only the region near the surface become particles, adjacent to which there is a flat region. 
Fig. 1. TEM cross sectional image of an etched Fe-Si
catalyst. It is noted that the thickness of the catalyst
layer increases from 24 nm for its as-deposited state to 42 nm for
its etched state. The flat region alone has a thickness of 12
nm. The catalyst obviously swells after the hydrogen
bombardment during the etching. The surface of Al/Fe-Si
catalyst also becomes particle-like after the hydrogen etching as
mentioned above. However, the swelling is more severe due to
the addition of an Al interlayer. For example, the thickness
increases from 2 nm (Al) + 24 nm (Fe-Si) = 26 nm for its
as-deposited state to 46 nm for its etched state. Notably is
that the flat region alone has a thickness of 18 nm, which is almost
50% thicker than that of etched Fe-Si catalyst, i.e., 12 nm as
mentioned above. Also, the Al interlayer is no longer
seen. The Al is not seen in these images as it has diffused
into the Fe-Si. Al was found to diffuse up to a depth of 35nm
into the Fe-Si film. Most of the Al atoms were diffused over a
region of ~20 nm, limited between ~18 nm and 38 nm. For
Al/Fe-Si having thicker Al interlayers, i.e., 4 nm, 6 nm, 8 nm, and
12 nm, the cross sectional morphologies and microstructure are
similar to that of 2-nm-Al/Fe-Si except that the Al interlayers are
visible in the 6-nm-Al, 8-nm-Al, and 12-nm-Al/Fe-Si catalysts.
Fig. 2. SEM cross-sectional images of CNTs grown on
etched Fe-Si catalysts having (A) 0-nm, (B) 2-nm, (C) 8-nm, and (D)
12-nm Al interlayers. Figs. 2A, 2B, 2C, and 2D show SEM cross-sectional images of CNTs grown on various etched Fe-Si catalysts having 0-nm, 2-nm, 8-nm, and 12-nm Al interlayers, respectively. The methane/hydrogen ratio was 4/9. 
Fig. 3. CNT length increases and then decreases with the
Al interlayer thickness. The methane/hydrogen ratio was
4/9. The relationship between the CNT length and the Al
interlayer thickness is further shown in Fig. 3. The length of
CNT increases and then decreases with the thickness of the Al
interlayer. In particular, the average lengths of the CNTs
grown on the Al/Fe-Si catalysts having ~3±1 nm think Al interlayers
are more than 3 times longer than that grown on the Al/Fe-Si
catalysts having no Al or thicker Al interlayers. Similar
effects of Al interlayer were also observed at lower
methane/hydrogen ratios of 3/9, 2/9, and 1/9. Apparently there
is an optimal thickness range of Al interlayer that greatly enhances
the growth of CNTs. Furthermore, within this thickness range,
the use of an Al interlayer is in fact so effective such that
normally required catalyst etching is waived. Without the
hydrogen etching, no CNT growth was observed when as-deposited Fe-Si
thin films are used as the catalyst. Less effective etching of
a catalyst normally occurs during the early stage of CNT
growth. This allows the aforementioned swelling of the
catalyst to take place and therefore the growth is enhanced.
The enhanced CNT growth due to the use of a very thin Al interlayer
is explained by considering the structural changes of the catalyst
as mentioned above.Fig. 4 shows the Raman spectra of CNTs grown on etched Fe-Si, as-deposited 2-nm-Al/Fe-Si, 
Fig. 4. Raman spectra of CNTs grown on etched Fe-Si,
as-deposited Al/Fe-Si, and etched Al/Fe-Si. The
methane/hydrogen ratio was 4/9. and etched 2-nm-Al/Fe-Si at a
methane concentration of 4/9. The Raman signatures spectra are
in general similar. Commonly observed D-band and G-band are
seen. However, their ID/IG ratios suggest that these CNTs
contains different amount of defects. The CNTs grown on the
un-etched or as-deposited catalyst of Fe-Si contain the highest
amount of such disorders (ID/IG = 1.95); while the defect levels of
the CNTs grown on etched catalysts of both Fe-Si (ID/IG = 1.50) and 2-nm-Al/Fe-Si
(ID/IG = 1.55) are less and similar.
In other words, the presence of a thin layer of Al greatly enhances
the growth rate of CNTs while maintaining similar microstructures
for the resulting CNTs.Conclusions Vertically aligned CNTs have been synthesized using Fe-Si catalysts deposited on Si substrates with and without an Al underlayer at a low temperature of 370 °C. The average lengths of the CNTs grown on Al/Fe-Si catalysts having ~3±1 nm thick Al are more than 3 times higher than that grown on Al/Fe-Si catalysts having no Al or thicker Al. Within this thickness range, the use of an Al interlayer is in fact so effective such that normally required catalyst etching is waived. Such beneficial effects were observed regardless of the methane concentration used. The effects are attributed to the diffusion of appropriate amount of Al into Fe-Si catalyst which causes swelling of the catalyst, i.e., making the catalyst porous. Also, the presence of a thin layer of Al greatly enhances the growth rate of CNTs while maintaining similar microstructures for the resulting CNTs. References
|
||||
|
||||
 | ||||
