22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Advanced nanoscale engineering and devices fabrication with atmospheric
pressure microplasmas
D. Mariotti
Nanotechnology & Integrated Bio-Engineering Centre (NIBEC), Ulster University, Newtonabbey, U.K.
Abstract: Microplasma synthesis and their materials processing capabilities are discussed.
A range of materials and structures are exemplified and some of their tuneable properties
reported. The integration of microplasma-based processes to fabricate and potentially
manufacture future technological devices (e.g., solar cells) is then analysed.
Keywords: atmospheric pressure, microplasmas, nanomaterials, solar cells, quantum
confined nanoparticles
1. Introduction
Atmospheric pressure microplasmas have demonstrated
unprecedented versatility for the synthesis and processing
of nanoscale structures [1-3]. Microplasmas have shown
the possibility of producing thin films, nanostructured
coatings, nanoparticles and other complex materials from
a variety of precursors that include solid, liquid and gases.
The
synthesis
of
metallic,
metal-oxide
and
semiconducting materials has been demonstrated covering
a wide range of elements in the chemical table. Despite
research efforts go back only a decade, the quality of the
materials produced is in many cases comparable to results
produced with other methods (e.g., wet chemistry, lowpressure plasmas) [4].
Most recently, microplasmas at atmospheric pressure
have revealed great opportunities for nanoscale
engineering, providing unique avenues for accurately
tailoring materials properties. While the scale-up of
atmospheric pressure microplasmas has not been
demonstrated yet, progress has been made also in this
direction which suggests the possibility of integrating
microplasma processes in the fabrication of application
devices.
In this contribution we will first review the capabilities
of microplasma-based materials synthesis, highlighting
the wide range of achievable morphologies and chemical
compositions.
Following, examples of advanced
nanoscale engineering will be provided that include
synthesis of alloyed nanoparticles, surface engineering of
quantum-confined nanoparticles and the formation of
hybrid organic/inorganic nanocomposite structures. The
possibility of introducing microplasma-based processes in
the fabrication of solar cell devices (within a research
setting) is then discussed and finally the challenges
associated with scaling up atmospheric pressure
microplasmas will be analysed.
2. Synthesis of alloyed and composite quantumconfined nanoparticles
SiC quantum confined nanoparticles (diameter < 5 nm)
could be synthesized by an atmospheric pressure
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microplasma system (Fig. 1a-b). The plasma was
generated within a rectangular glass tubing with an
internal cross section of 0.5 mm x 5 mm (0.3 mm wall
thickness). Two identical copper electrodes were placed
at both sides of the rectangular glass tubing. The plasma
is sustained by radio frequency (RF) power at 13.56 MHz.
The gas flow direction is along the longer length of the
plasma, i.e., along the 20 mm long electrode side. The
background gas for sustaining the plasma is argon.
Tetramethylsilane (TMS) was used as precursor and was
delivered to the reactor using a bubbler with a given flow
of argon. The total gas flow rate through the reactor is
maintained constant at 1000 sccm and the applied RF
power is fixed at 100 W. The bubbler was used to control
the flow and concentration of TMS in the plasma, and
consequently, the size of the NPs.
Fig. 1. Microplasma reactor used for the synthesis of
elemental silicon/silicon-carbide nanoparticles: a) photo
of the reactor, and b) close up with the microplasma
turned on. c) Diagram depicting the direct deposition of
elemental Si nanoparticles (NPs) for full device
fabrication. ITO stands for indium-tin-oxide.
Transmission electron microscopy (TEM) analysis has
shown that NPs with 1.5 nm, 3.7 nm and 5.3 nm average
diameters could be produced with 0.4 sccm, 2.4 sccm, and
5 sccm TMS flow, respectively. The crystalline structure
and chemical composition was confirmed by TEM, X-ray
1
photoelectron spectroscopy (XPS) and Fourier transform
infra-red spectroscopy (FTIR).
These results demonstrate the ability of this simple
system to produce high quality quantum-confined alloyed
NPs. By selecting appropriate precursors, the process is
easily applicable to a range of other materials and for
instance ligand-free elemental silicon and Sn-SiC
nanoparticles (NPs) are also synthesized both directly in
colloids as well as in films. Other composite and alloyed
semiconducting materials are being investigated.
3. Advanced surface engineering by microplasmas
Quantum confined NPs can be manipulated to tailor
their energy structure and transition dynamics: for
instance modifying the NPs surface it is possible to vary
absorption, carrier life-time, stability and overall
optoelectronic properties [5, 6]. Microplasma-liquid
interactions
have
recently
revealed
interesting
opportunities for surface engineering of quantum confined
NPs directly in colloids [5, 7]. Microplasmas can be
generated above the colloid either by directly coupling the
plasma with direct-current or configured as a plasma jet
impinging on the surface of the colloid [8].
A microplasma-liquid system has been used to modify
the surface characteristics of silicon NPs, converting
H-terminations in short organic ligands [7] as well as
forming a thin oxide-based shell [8]. The stability and
photoluminescence properties have been monitored and
show drastic improvements with the possibility of varying
the emission wavelength [5, 7, 8].
The same method is also being applied for surface
engineering of a wide range of other materials that
include Au, TiO 2 , BN, SiSn NPs. In all cases, the surface
treatment results in better dispersion and passivation with
great advantages for corresponding applications. Some
initial results are reported for the improvement of the
optical properties of SiSn NPs for photovoltaic
applications and the dispersion of BN NPs to be used as a
flame retardant.
Plasma-liquid systems have also allowed the one-step
fabrication of organic/inorganic nanocomposites that are
not easily achieved with other techniques. We have been
able to produce core/shell Au NPs/polymer structures
with a one-step fabrication process which have important
biomedical applications. We have also produced Si
NPs/polymer nanocomposites and demonstrated their
integration in solar cell devices. We have therefore
demonstrated ~10% efficiency improvements in organic
solar cells.
deposit films of Si NPs on experimental third generation
solar cell devices. Figs. 1a and 1b are photos of the
microplasma reactor, while Fig. 1c depicts the reactor
depositing Si NPs films on a glass substrate with
patterned indium-tin oxide. Following the deposition of
the Si NPs layer, the experimental device is completed
(bottom of Fig. 1c).
Microplasmas are already capable of depositing most of
the materials required for third generation solar cell
device fabrication; it is believed that with the progress of
these types of solar cells, microplasma scale-up will also
be possible for a full device fabrication process that
heavily rely on atmospheric pressure plasma processes.
5. Acknowledgements
This work was supported by the Royal Society
(IE120884), EPSRC (EP/K022237/1), the Leverhulme
Trust (IN-2012-136) and EU-FP7 (RAPID- 606889).
6. References
[1] D. Mariotti and R.M. Sankaran. J. Phys. D: Appl.
Phys., 43, 323001 (2010)
[2] D. Mariotti and R.M. Sankaran. J. Phys. D: Appl.
Phys., 44, 174023 (2011)
[3] D. Mariotti, A.C. Bose and K. Ostrikov. IEEE
Trans. Plasma Sci., 37, 1027 (2009)
[4] S. Askari, I. Levchenko, K. Ostrikov, P. Maguire
and D. Mariotti. Appl. Phys. Lett., 104, 163103
(2014)
[5] D. Mariotti, S. Mitra and V. Švrček. Nanoscale, 5,
1385 (2013)
[6] V. Švrček, M. Kondo, K. Kalia and D. Mariotti.
Chem. Phys. Lett., 224, 478 (2009)
[7] V. Švrček, D. Mariotti and M. Kondo. Appl. Phys.
Lett., 97, 161502 (2010)
[8] S. Mitra, V. Švrček, D. Mariotti, T. Velusamy,
K. Matsubara and M. Kondo. Plasma Process.
Polymers, 11, 158 (2014)
4. Photovoltaic devices produced by atmospheric
pressure plasmas, is it possible?
The focus of our research is on the development of third
generation photovoltaic devices.
We have so far
integrated microplasma processes for the deposition of
molybdenum oxide (as transport layer) and Si NPs as
active layer.
Fig. 1 shows the microplasma reactor that is used to
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