22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Nanoparticle surface termination as a temperature probe
T. Lopez1 and L. Mangolini1,2
1
2
Mechanical Engineering Department, UC Riverside, Riverside CA, U.S.A.
Materials Science and Engineering Program, UC Riverside, Riverside, CA, U.S.A.
Abstract: Non-thermal plasmas have been shown to heat particles to temperatures that
well exceed the temperature of the surrounding carrier gases. We have measured the
temperature of silicon nanoparticles produced in continuous flow non-thermal plasma
reactor by monitoring their surface termination. In-situ, in-flight FTIR has been utilized to
track changes in the surface termination of particles as they are suspended in the partially
ionized gas. Correlation with the kinetics of thermally-induced hydrogen desorption allows
obtaining the particle temperature as a function of process parameters.
Keywords: Silicon, Particle Temperature, Surface Chemistry, Fourier Transform Infrared
Spectroscopy (FTIR)
1. Introduction
The correlation between plasma input parameters and
the resulting particle size [1], structure [2, 3], and surface
termination [2, 3] has allowed the community to reach a
deeper understanding of the physical and chemical
phenomena responsible for the production of nanoparticle
in non-thermal plasma reactors. This understanding is
expected to enable the use of plasma-produced
nanomaterials for many applications, including for
nanoelectronics [4], light emitting devices [5, 6],
photovoltaics [7] and, thermal electric devices [8].
Particle size, structure, and surface termination are all
particle properties that are directly correlated to the
particle interactions with ions and other plasma produced
radicals [9]. It has been shown that any combination of
these properties can be obtained and controlled separately
during particle production [1, 3]. Varying the pressure in
the plasma reactor controls the residence time in the
plasma, i.e. allowing for nanoparticle size control [1].
Structure has been shown to be a function of plasma input
power and in-flight annealing temperature [1, 3]. While
surface termination is first dependent upon choice of
precursor (SiH 4 leads to hydrogen termination while
SiCl 4 leads to a chlorine termination), it has also been
shown to have a dependence on in-flight annealing
temperatures. In-flight annealing of continuous flow nonthermal plasma produced particles can produce near bare
surfaces of produced silicon particles [1, 3]. Moreover,
nucleation, growth, and crystallization are all dependent
on the phenomena occurring during interaction with the
plasma. Crystallization in particular may be strongly
dependent on the heating of particles in non-thermal
plasmas [1, 3, 9]. Although these previous endeavours
have had the ability to estimate a temperature a particle
must achieve for nucleation or crystallization to occur
within a plasma, there has yet to be an empirical
measurement of particle temperature during interaction
with the plasma. This contribution details an empirical
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in-situ measurement of particle temperature as a function
of plasma power input, by ways of monitoring surface
morphology via in-situ FTIR.
2. Experimental
A continuous flow non-thermal plasma reactor
modified specifically for in-situ FTIR absorbance
measurements has been utilized to monitor particle
surface termination due to plasma interactions. A
schematic of this reactor is shown in figure 1. The reactor
utilized in this experiment is comprised of two separate
Figure 1: Modified continuous flow non-thermal plasma
reactor optimized for in-situ FTIR measurements.
continuous flow non-thermal plasma reactors. The first is
a 3/8” tube 6” long that flares out to 1”. It is then
connected to a 1” quartz tube that is offset by 900. The
primary plasma is ran with 70 sccm SiH 4 (1.38% silane in
Ar), 30 sccm Ar, and 5 sccm H 2 . The reactor uses a dual
ring electrode setup, utilizing a 13.56 MHz power supply,
and a T-matching network allowing for a capacitive
coupled plasma to be struck and maintained. The power
supply is set to 30 watts and the pressure is at 3.5 Torr.
These conditions create crystalline silicon nanoparticles
that are 5-10 nm in size and with hydrogen terminated
1
surface. The second plasma is offset from the primary
plasma by 900 it is fused together at the end of the
primary plasma reactor’s 1” tube. The second reactor is
30” long, 20” of which is utilized for the secondary
plasma. A parallel plate electrode setup is utilized in the
secondary plasma, this allows for a very uniform plasma
to be maintained throughout the length of the reactor.
The secondary plasma is comprised of the remaining
argon and hydrogen that flow through the system along
with the silicon nanoparticles produced in the primary
plasma. The ability to create a stable 20” long plasma
allows achieving a sufficiently long optical path for the
in-flight IR detection of the silicon particles. The
secondary plasma uses a separate power supply and Tmatching network, which allows controlling the power in
the secondary plasma completely independently from the
nanoparticle-producing step.
3. Results and Discussion
Kramer et al.[10] have shown that an increase in input
power in a plasma leads to an increase in the production
of argon ions and atomic hydrogen, leading to an increase
in particle temperature (see inset of figure 1). In our
experiment we control the electrical power input to the
second plasma while monitoring the amount of hydrogen
present at the surface of the particles. It is well established
for many different systems, i.e. nanoparticles [9] and thin
films [11], that the amount of hydrogen left on a
particular surface is dependent upon the surface
temperature, as shown in figure 2. In figure 2A we show
the FTIR spectra for a porous silicon thin film heated to
different temperatures. Figure 2B and 2C show the
integral of the intensity of the FTIR spectra associated
with the vibrational modes of the surface hydrogen. In
both cases the broad absorption cantered at 2080cm-1 and
980 cm-1 are representative of the different vibrational
modes of the for SiH x , where x=1,2,and 3.
As the temperature increases the amount of hydrogen
left on the surface decreases (figure 2B). In previous
works [2, 9] we have shown very similar results by
performing in-situ FTIR measurements of in-flight
annealed nanoparticles (figure 2C). In more recent work
we have resolved the surface termination of particles
while they are suspended in a plasma (see figure 3),
revealing that a correlated measurement of particle
temperature could be made based on particle surface
termination.
It is known that by increasing temperature the value of
the integral intensity decreases [9, 11], as shown in figure
2B and 2C. By comparing FTIR absorption spectrum
collected from the plasma to those collected previously in
annealing studies and from the literature a direct
correlation of surface termination to particle temperature
can be made. Furthermore, by taking FTIR absorbance
measurements of the particles as they are flowing through
the second plasma allows for an empirical measurement
of the particles temperature as a function of plasma input
power.
In figure 3 we show the spectrum of the gases and
particles flowing out of the primary plasma, with no
secondary plasma running. This spectrum shows that all
of the silane from the primary plasma has been consumed
and converted into particles. Moreover, a clear broad IR
absorption feature at 2180 cm-1 due to hydrogen surface
termination is present, inset figure 3.
Figure 3: FTIR Absorbance of particles without
secondary plasma. Inset: Broad absorbance centered at
2080 cm-1 associated with the SiHx (x=1,2,3) vibrational
modes of hydrogen terminated silicon
Figure 2: (A) FTIR of porous hydrogen terminated
thin film [11]. (B) Integral area of the peaks shown in
(A). (C) Integral area for the case of in-flight annealed
hydrogen terminated silicon nanoparticles.
2
Figure 4 shows three IR absorption spectra taken at
different secondary plasma powers. As one can see, the
hydrogen surface termination is decreasing as the plasma
power input is increasing. As plasma power increases the
amount of ions and radicals within the plasma is expected
to increase, leading to more intense nanoparticle heating.
These measurements suggest that the amount of heating
may be sufficient to lead to efficient desorption of
hydrogen from the nanoparticle surface, consistent with
the decreased absorption from surface silicon hydrides in
the FTIR spectra.
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Figure 3: FTIR Absorbance of particles in secondary
plasma for powers of 15, 20, and 30 watts.
4. Conclusion
Preliminary data shows that changes in the hydrogen
termination can be observed as silicon nanoparticles flow
through a plasma, as a function of electrical power input.
The data suggests that the amount of hydrogen coverage
of these particles is dependent upon the plasma power,
with higher power leading to a decrease in hydrogen
surface coverage.
We tentatively interpret this
observation in terms of heating of the particles while in
the plasma. Sufficient particle heating that leads to
thermal desorption of hydrogen is likely occurring within
the plasma. A complete characterization of the
nanoparticle-plasma interaction based on the data
acquired using such experimental set-up will be presented
and discussed.
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5. References
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Lett. 5, 655 (2005).
[2] O. Yasar-Inceoglu, et al., Nanotechnology 23, 255604
(2012)
[3] Lopez, T., et al. Nanoscale, 2014. 6.3 p.1286-1294
[4] S. Oda, Materials Science and Engineering, 2003,
B101, 19-23.
[5] K.-S. Cho, et al., Shin, Applied Physics Letters, 2005,
86, 071909.
[6] K.-Y. Cheng, et al., Nano Letters, 2011, 11, 19521956.
[7] C.-Y. Liu, Z. C. Holman and U. R. Kortshagen, Nano
letters, 2009, 9, 449-452
[8] Garay, J. E. Annual review of materials research 40
(2010): 445-468.
[9] Lopez, Thomas, and Lorenzo Mangolini. Journal of
Vacuum Science & Technology B 32.6 (2014): 061802.
[10] N. J. Kramer, R. J. Anthony, M. Mamunuru, E. S.
Aydil, and U. R. Kortshagen, J. Phys. D: Appl. Phys. 47,
075202 (2014).
[11] P. Grupta, et al., Physical Review B, vol. 37, 14
(1988)
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