1) The author synthesized silicon-doped carbon nanotubes (CNTs) catalyzed by a nickel thin film via chemical vapor deposition. Nickel films were deposited on silicon wafers and shown to catalyze the growth of silicon nanowires when annealed at high temperatures. 2) Scanning electron microscopy images showed the successful growth of silicon nanowires through nickel films in argon and nitrogen environments but not in oxygen. This process may occur via a vapor-liquid-solid mechanism. 3) Further experimentation introducing a carbon source during growth may allow for the synthesis of silicon-doped CNTs, which could improve battery performance and energy storage if achieved. The nickel film method provides a

1. Synthesis of Si-Doped CNTs Catalyzed by a Ni Thin Film via Chemical Vapor Deposition
Bradley J. Sugg
Chemistry 491 - Spring 2015
Department of Chemistry
Central Michigan University
In fulfillment for CHM 491 research
under the supervision of Dr. Bradley D. Fahlman

2. Abstract:
The synthesis of silicon doped carbon nanotubes (CNT’s) is something that could be applied to
many different applications across the board. It has been noted that silicon nanowires are able to grow
through a thin film of Ni (5-10 nm), providing a viable method for its synthesis. It was found that silicon
nanowires were able to grow through this film via a possible vapor-liquid-solid, VLS, model5
. While the
exact mechanism of this formation is still unclear, it was evident that the possibility for this was there.
Experimentally, the evidence suggests that nickel is used to catalyze the growth of CNT’s6
, while
incorporating Si inside the walls of the nanostructure. If a process is discovered for doping CNT’s, could
possibly mean the replacement of a batteries anode component. Accomplishing this would result in an
increased amount of time between charges for electrical systems containing Si doped CNT as the anode
material.
Introduction:
After the discovery of buckminsterfullerene, C60, in 19851
, the field of research for the
development of Nanostructures has seen major advances. Due to the unique properties exhibited by
these compounds there has been a great deal of interest. Korto et al. was the first to exhibit the growth
of these spherical carbon structures while trying to understand the mechanism behind the formation of
long carbon chains, found in space1
. Much research involving the doping of these structures has taken
place. However, the extensive network of labile pi-bonds2
has proven this incorporation to be difficult.
One of the main issues was the total loss of the fullerene structure. Ray et al. in 1998 reported the
successful synthesis of 2 silicon atoms as a part of the C60 structure with promising results for the
stability of the compound. The discovery of the first carbon nanotubes, CNT, was reported in 19913
. It
has sense been a goal to continue the development of the CNT structure as well as its properties. One
of the fastest growing markets for this compound is in the field of battery research, as developing
methods for doping CNT with silicon could mean the potential for the replacement of the anode in all
modern batteries. The structures of these CNT comprise a large surface area, meaning that there is
more surface space for Lithium ions to bind to. As a result of the metalloid properties of Si, their doping
onto CNT’s could also revolutionize the field of solar energy storage. This technology has the potential
to increase the battery life across the board, from electric cars to apple watches.

3. Experimental Procedure:
1. Deposition of Ni Film:
The reagent that was used for the deposition of nickel was nickel acetylacetonate. This
substance was in a powdered form and bright green in color. The method that was used to deposit this
film was chemical vapor deposition4
. The initial steps for this procedure included preparing a small
piece of pure silicon wafer. The average size that was used for this step was no larger than the size of a
quarter. From here, the silicon wafer was then subjected to an acetone wash in order to remove any
impurities. The wafer was then placed inside of a quartz tube that ran through the CVD. The precursor
was then weighed inside of a ceramic weigh boat which was positioned inside the initial portion of the
tube, while still remaining outside of the CVD. Two gas lines were then attached to the beginning of the
quartz tube with one nozzle positioned just before the nickel acetylacetonate. This nozzle provided
argon, which acted as the carrier gas. The second nozzle extended overtop of the nickel precursor and
continued until just before the furnace. This nozzle supplied the hydrogen gas, which will reduce the
nickel particles being carried by the argon, on the substrate. Both ends of the tube were then closed off
via an air tight clamp. To ensure that no oxygen remained inside of the quartz tube, argon was allowed
to flow at a rate of 300 sccm, 2 minutes prior to heating. While purging out the oxygen, high
temperature heat tape was then wrapped around the portion of the tube that housed the nickel
acetylacetonate. This heat tape was then hooked up to a variac transformer. Once everything was
hooked up, the silicon substrate was heated to 265 °C, while the temperature of the heat tape was
adjusted to 220 °C. This reaction was run for one hour. After that, the CVD and variac were turned off
and allowed to cool to room temperature. It is important to note that after the hour trial was
completed, the flow of hydrogen was stopped, while argon remained flowing at its initial rate. Once the
wafer was removed from the tube, it was subjected to another acetone wash to clean and investigate
how well the film adhered to the surface. A schematic of this procedural setup is provided as figure #1
below.

4. Figure 1: Showing the experimental set-up for the deposition of Ni atop of a Si wafer. The portion labeled A denoted the area of
tubing that was wrapped with high temperature heat tape.
2. Annealing silicon nanowires
For this process, the use of CVD was employed in order to achieve a high reaction temperature.
The experimental set up was nearly the same as seen for the deposition of the film with some minor
changes. The silicon wafer coated with the thin nickel film, was then placed inside of a quartz tube that
ran through the furnace. This wafer was position in the middle of the furnace to ensure that the
reaction temperatures remained constant thought the annealing process. A flow meter was then
hooked up to the quartz tube in order to allow the flow of argon to remain constant, at 110 sccm. Once
it was believed that only argon was present in the tube was when the furnace was turned on. For this
process, the reaction temperature was set to 1100 °C and was ran for 20 minutes. Afterwards, the
furnace was turned off while argon still remained flowing at its reaction rate. Another two trials where
also conducted using all of the same parameters as above. The only change was the use of nitrogen gas
at a flow rate around 230 sccm and annealing in an environment saturated with oxygen. Once either
one of the two trails were cooled down to room temperature, they were then carefully removed from
the tubing and place in an environment with limited exposure to oxygen to retard any oxidation
Results:
To validate the potential for this research, two initial trials were completed. This information
would guide whether the use of a sputter film, or a dried solution, atop a Si wafer would yield Si
nanowires. The composition of the solution and film was a mix of Au-Pd, 60-40, whose thickness was 10
nm. The procedure that was used to anneal the Si nanowires is outlined in section 2 of the experimental
procedure. These wafers were then analyzed via the use of the Hitachi S-3400N scanning electron

5. microscope. Figure #2 (a) shows the Si wafer with the Au-Pd sputtered film, while figure #2 (b) shows
the wafer that contained the Au-Pd solution.
A) B)
Figure 2: (A) SiNW grown through an Au-Pd (60:40) 10 nm, sputtered film; (B) SiNW grown from dried Au-Pd solution.
Through the initial experiments, it was concluded that using a nickel film was the best choice for
the growth of nanowires. This process was outlined through a paper written by Maruyama et al. by
heating the nickel precursor, Ni (acac)2, to a temperature that would allow the nickel to sublime. Once in
its vapor phase, the nickel was then carried via the argon into the tube furnace. From here, the nickel
would then be deposited on the Si wafer by the hydrogen reduction of the vapor phase nickel. It was
found that there was a film that was being deposited, as a result of the wafer surface becoming
increasingly opaque. These observations where then confirmed through analysis of the surface via film-
metric spectroscopy. It was found that the coverage of the film was not uniform thought the surface of
the wafer, but the fit remained consistently good. Several parameters, including the total flow rate of
either gas, amount of nickel precursor, and reaction time where adjusted throughout several trails for
the sake of optimization. It was found that through the adjustment of the flow rate for either hydrogen
or argon did not greatly affect the thickness of the Ni thin film. Instead, by running this reaction with a
greater amount precursor, ≥3 g, had the greatest influence on the thickness of the film. It should be
noted that there is a proportional relationship between the initial amount of Ni (acac)2 and the overall
reaction time. Using this technique, yielded film thicknesses that varied from ≥1 nm to 9 nm; all also
exhibited an excellent fit. These samples were then analyzed using Energy-dispersion X-ray
spectroscopy, EDS, in order to find the composition of the material deposited on the surface. Figure #3
and #4 show the EDS analysis of a nickel film that was reported as 5.5-6 nm and 1-1.5 nm, respectively,
through film-metrics spectroscopy.

6. Figure 3: EDS spectrum of 5.5-6 nm thick Ni film.
Figure 4: EDS spectrum of 1-1.5 nm thick Ni film
From here, the electrical conductivity of 3 Ni films where tested. It was found that the resistivity
of the nickel film was close to the value found for bulk nickel, which is found as 6.99E-8 Ω·m4
. Table 1
shows the data that was used to calculate the resistivity as well as the resistance.
Table 1: The data used to find the Resistivity 3 separate nickel films all with different thicknesses tested at room temperature.
No: Film Thickness (nm) Resistance (Ω) Area (m2
) Length (m) ρ (Ω·m)
1 1.25 1417 1E-12 0.02 5E-08
2 3.30 318062 5E-12 0.03 5E-05
3 8.50 1586 2E-13 0.01 3E-08

7. From this step it was then the goal to anneal silicon nanowires through this nickel film, as seen
with the initial trials contain the Au-Pd mix. This was conducted in three separate, experimental
environments. The first environment was using an open air furnace. This means that, unlike the tube
furnace, the sample was surrounded by oxygen. This sample was then ran for 15 minutes at 1100 °C, it
was then allowed to cool overnight. Figure #5 shows the result as analyzed by a scanning electron
microscope.
Figure 5: SEM image of Ni nanoparticles after first attempt for annealing SiNW in an O2 rich environment.
The second environment was used in a tube furnace with argon flowing at a rate of 110 sccm.
The temperature of this reaction was at 1100 °C, and reaction time was for 15 minutes. Figure #6 shows
the sample as seen through a SEM.
Figure 6: SiNW's grown in tube furnace with flow of argon present.
The Final environment that was employed it attempts to grow silicon nanowires was in a tube
furnace. The gas used for this trial was house nitrogen, which was regulated via a flow meter at a rate of
230 sccm. Figure #7 show this sample under SEM.

8. Figure 7: All pictures are from the same sample grown under flow of nitrogen, only different in magnification.
Discussion:
From these results it can be the concluded that silicon nanowires were clearly formed. Analysis
that was completed through the use of EDS showed very sharp peaks at 1 keV signaling that the Ni vapor
was reduced via hydrogen over top of the silicon wafer. This result was also confirmed from the
consistently good fit that was calculated for nickel, through the use of the film-metric spectroscopy
software. Through these results, it can be said that there was in fact silicon nanowires present, as seen
in figure #6 and #7. It made no difference whether this formation took place in either nitrogen or argon
at temperature reaching 1100 °C. It was interesting to see that no growth was seen in the presence of
stagnate oxygen. This may have been the result of the increasing vapor pressure of oxygen when hot.
To further test this reasoning, more experiments would have to be run, where the O2 flows at the same
rate of the N2 and Ar. In figure #7, it can be seen that silicon nano-rod structures have formed as a
result of the Ni thin film. As a result of the Ni film deposition method having caused inconsistences in
the thickness of the Ni film. These rods may have grown on areas that are thicker or thinner than
others. Further tests are needed to be run, in order to determine if the synthesis of Si doped CNT’s
using the outline of this report would be a viable option. Experimentally it’s suggested to introduce a
carbon source, such as acetylene or methane, during the period when the growth of SiNW’s was
exhibited. If this method does not work then there are plenty of others to try.

9. The mechanism for the growth of these SiNW, as well as CNT’s, is still not known but it is
suggested that this may occur via a vapor-liquid-solid (VLS) model5
. This can model can be seen in figure
#8.
Figure 8: This shows the possible growth mechanism for SiNW via the VLS model6.
This occurs when the catalyst, Ni, liquefies as a result of high temperatures. This catalyst then
become the preferred site of nucleation for the gaseous reagent. Growth is seen when the Ni liquid
becomes super saturated and which then results in the precipitation of the nanostructure via the
bottom up model6
.
Conclusion:
The idea of doping carbon nanotubes with Si through a thin film (5-10 nm) of Ni are something
that may need a little more experimentation in order to properly demonstrate. The experimentation
that was done along with this lab report supports the hypothesis of this being a possible option. It was
shown here that it was possible to facilitate SiNW growth through a thin metal film. Nickel films have
been reported to catalyze the growth of CNT’s which shows that precedence for this hypothesis. It was
also hypothesized the VLS mechanism may be how these nanostructures are formed. The implications
that this type of research has is of major importance. With the development of new technology will
bring with it the need for batteries that can power complex machine. Through the use of Si doped
CNT’s, this need may very well become a reality in the near future.
References:

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Nature. 1985, 318, 162-163.
2. Mazumder, B. Silicon and its Compounds; Science Publishers, Inc: Enfield (NH), 2000, Chapter
5.
3. Iijima, S.; Helical microtubules of graphitic carbon. Nature. 1991, 354, 56-58.
4. Maruyama, T.;Tago,T. Nickel thin films prepared by chemical vapour deposition from nickel
acetylacetonate. J.of Materials Science 1993, 28, 5345-5348.
5. Fahlman, B. D. Materials Chemistry, second ed.; Springer: New York, 2011, Chapter 6.
6. Feng, J. L.; Zhang, S.; Kong, H. J.; Guo, J.;Cao, B. X.; Li, B. Growth of crystalline silicon
nanowires on nickel-coated silicon wafer beneath sputtered amorphous carbon. Thin solid
Films.2013, 534, 90-99.