22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
High quality tin quantum dots: synthesis and direct incorporation in SiC
quantum dots by atmospheric pressure microplasma
A. Ul Haq1, S. Askari1, P. Maguire1, V. Svrcek2 and D. Mariotti1
1
2
Nanotechnology & Integrated Bio-Engineering Centre, Ulster University, BT37 0QB, U.K.
Research Centre for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology
(AIST), Tsukuba, Japan
Abstract: A straightforward and economical method based on atmospheric pressure
microplasma was developed to synthesize tin quantum dots. The size of the quantum dots
resulted to be in the range of 1.1-2.4 nm with an average size of 1.6 nm. Transmission
electron microscopy analysis confirms the crystalline nature of these quantum dots that can
be collected in liquid and or on any substrate. Furthermore, incorporation of tin in silicon
carbide quantum dots resulted in reduction of its optical band-gap. Such quantum dots are
highly demandable for optoelectronic applications and energy storing devices.
Keywords: quantum dots, microplasma reactor, atmospheric pressure, band-gap
1. Introduction
Quantum dots (QDs) or nanocrystals (NCs) are crystals
with diameters that are smaller than the corresponding
material exciton Bohr atomic radius [1, 2] and are very
attractive for future optoelectronic applications. The
rapidly increasing trend of research in NCs is due to their
unique quantum confinement effects. Various types of
NCs have been employed in energy conversion and
storage applications, however toxic elements such as Pb,
As and Cd are often considered in particular for
photovoltaic devices [3]. Group IV NCs and their alloys
represent low-cost and environmentally friendly
alternative materials that may offer great potential
towards commercializing efficient, cheaper and
sustainable devices for opto-electronics and energy
storage.
Tin (Sn) and in particular (α-Sn), a zero direct band-gap
semiconductor [4, 5], belongs to the same group of Si and
promises interesting properties which are very crucial in
enhancing energy storage. The use of Sn nanoparticles as
an anode material for lithium-ion batteries has been
reported [6-8]. Although the theoretical energy capacity
of Sn is very high (990 mAh/g), yet their performance in
lithium ion batteries has been so far poor due to the
material degradation originating from volume expansion
[8]. Volume expansion arises from the long diffusion path
lengths for Li-ions inside the tin nanoparticles [8]. One
possibility for improving device performance is therefore
to shorten the Li-ion intercalation path by using smaller
tin nanoparticles (5 nm). Unfortunately, most of the
synthesis techniques yield Sn NCs with diameters larger
than 5 nm [7, 9-10]. The difficulty in synthesizing Sn
NCs arises also because of the tendency to agglomerate,
which worsen as the size of the NCs is reduced.
Furthermore, the diamond cubic structure offers lower
diffusion barriers for the intercalating ions as compared to
tetragonal structure (β-Sn) [13]. However, the stability of
P-II-5-8
bulk α-Sn, with diamond cubic structure at room
temperature is challenging due to the fact that structural
transformation to cubic-tetragonal occurs at around
13.2 oC [14]. For these reasons, Sn NCs have been
synthesized so far only within an embedding matrix or by
using organic capping agents [11, 12], which represent
clear transport challenges. Hence it is highly desirable to
look for a synthesis approach that can yield high quality
α-Sn NCs with diameters of less than 5 nm and that offer
improved stability and quality without stabilizing and
supporting agents.
Another attractive feature of tin is the possibility of
being incorporated in group IV indirect semiconductors
(like Si and SiC) in order to tune their band-gaps. Bandgap tuning, also called compositional tuning in this case,
can lead to transformation of the indirect nature of Si/SiC
NCs into direct band-gap NCs behavior [15].
Herein, we report on a straightforward microplasmabased method at atmospheric pressure for producing freestanding Sn NCs in a controllable size range (1-3 nm). Sn
NCs are produced by exposing sacrificial solid Sn wires
to the microplasma inside the plasma reactor and are then
collected in ethanol (or any substrate). This approach is
very simple, potentially scalable, safe, and relatively low
cost. Furthermore, using the same microplasma approach
but with different gaseous precursors, we have explored
the possibility of incorporating tin in SiC QDs with the
aim of modifying their optical properties. While these
represent very preliminary results, our findings are very
promising. The synthesized NCs were then characterized
by transmission electron microscopy (TEM: JEOL JEM2100F operated at 200 keV) which confirmed the NCs
crystallinity and structure. The size distribution of NCs
was obtained by image processing (ImagJ software) of the
TEM micrographs. The chemical analysis was carried out
by Fourier transform infra-red spectroscopy (FTIR)
equipped with an attenuated total reflectance (ATR)
1
accessory (Thermofisher iD5) while optical properties
were measured using an ultra-violet/visible (UV/VIS)
spectrometer (PerkinElmer Lambda 35).
2. Experimental Section
The experimental atmospheric pressure microplasma
reactor setup used for the synthesis of Sn NCs is
illustrated by a schematic diagram as shown in figure 1-a.
A Sn wire (0.25 mm diameter), which will act as a
sacrificial solid precursor, is inserted in a quartz capillary
tube (0.7 mm internal and 1 mm external diameter). The
plasma reactor consists of two ring electrodes around the
quartz capillary. One of the electrodes is grounded and the
other one is supplied with radio frequency power (40
watt) at 13.56 MHz for sustaining the microplasma. He
gas is used at an optimized flow of 1 sLm. The
microplasma reactor was housed within a chamber
maintained at atmospheric pressure and filled with N 2 .
driven synthesis by microplasma; NCs are then stabilized
due to their nanoscale dimension and surface effects [7,
10].
a
b
220
311
331
511
Figure 1: a) A schematic diagram depicting the
atmospheric pressure microplasma reactor used to
produce Sn nanocrystals; b) A digital photographic
representation of the plasma setup during the
synthesis stage.
The NCs formed within the reactor are then collected in
a cuvette containing a liquid (ethanol was used in this
case). The crystals can also be directly deposited on any
substrate. The photographic representation of the setup is
also shown in figure 1-b.
3. Results and Discussion
The TEM analysis of the Sn NCs is displayed in figure
2. The NCs appear to be nearly spherical and are well
separated (figure 2-a). NCs charging during plasma
synthesis induce their electrostatic repulsion and
contribute to non-agglomerated NCs.
In figure 2-b, selected area electron diffraction (SAED)
analysis confirms the crystal structure of Sn to be
diamond cubic (α-Sn) and the initial strong rings observed
in the corresponding diffraction pattern are indexed as
220, 311, 331 and 511. The presence of α-Sn at room
temperature is initially due to the non-equilibrium kinetic-
2
c
d311 = 0.19nm
Figure 2: a) Transmission electron micrographs
showing well separated Sn nanocrystals; b) Selected
area electron diffraction patterns of the nanocrystals;
c) High resolution transmission electron micrograph
revealing the crystalline nature of the nanocrystals
along with their lattice fringes measured to be 0.19
nm.
Furthermore, we could not observe the formation of any
oxides of tin. In order to further confirm the crystalline
P-II-5-8
nature and measure the lattice spacing between fringes,
high resolution TEM (HR-TEM) was performed (figure
2-c). The NCs were found to be highly crystalline and the
spacing between the lattice fringes was measured to be
around 0.19 nm corresponding to (311) planes.
The size distribution of the NCs is shown in figure 3 by
using a histogram. The size range of the NCs was varying
from 1.1 nm to 2.4 nm with an average value of 1.6 nm
determined by fitting the experimental data with a normal
distribution.
Our atmospheric plasma setup has been already
successful in synthesizing Si and SiC NCs [16, 17]. Here,
an initial effort has been made to incorporate Sn in SiC
NCs using Sn wire and tetramethylsilane (TMS) as
precursors. Ar gas was bubbled through a TMS container
and then fed to the plasma reactor. The flow of Ar
bubbled through TMS was kept at 2 sccm. Carrier
background argon gas was kept at 1 sLm. The results
produced were also compared with SiC NCs synthesized
following our previous work.
semiconductor with values normally ranging within 2-6
eV [18].
In order to measure the band-gaps in SiC and Sn-SiC
NCs, we produced the corresponding Tauc plots (inset in
figure 4-b) using the relation as given below [19];
1/𝑛
𝛼ℎ𝑣 = 𝐴�ℎ𝑣 − 𝐸𝑔 �
where ‘𝛼’ is absorption coefficient, ‘𝐴’ is constant, ‘ℎ𝑣’
is incident photon energy, and ‘𝑛’ is constant, for direct
transition 𝑛 is equal to 2 and for indirect transition 𝑛 is
1
equal to . In (𝛼ℎ𝑣)1/2 − (ℎ𝑣) plot, the intercept of the
2
extrapolated straight line at (𝛼ℎ𝑣) = 0 gives the optical
band-gap of the corresponding material. The band-gaps of
SiC and Sn-SiC NCs were found to be around 4.26 eV
and 3.96 eV respectively. The reduction in the band-gap
may be attributed to the effect of Sn in the NCs.
30
Dm = 1.60 nm
25
Counts
20
15
10
5
0
1.0
1.2
1.4
1.6
1.8
2.0
2.2
Diameter (nm)
2.4
2.6
Figure 3: Size distribution of Sn nanocrystals with an
average diameter (D m ) of 1.6 nm produced by our
atmospheric pressure microplasma. The red curve
represents the normal fit to the distribution.
SiC and Sn-SiC NCs are characterized by ATR-FTIR as
shown in Figure 4-a. SiC NCs (produced without the Sn
wire in the plasma, red curve in figure 4-a) are confirmed
by a strong peak at 825 cm-1 corresponding to Si-C bonds.
Other peaks contributing to Si-CH 3 , Si-CH 2 , Si-H x and
C-H x bending/stretching modes are also indicated and
labelled in the spectrum, which generally correspond to
surface terminations. For the NCs synthesized with the Sn
wire (black line, figure 4-a), there is a some evidence of
the presence of Sn due to Sn-O bond stretching vibration
that produces a peak at around 560 cm-1.
UV/VIS absorption of the Sn-SiC and SiC NCs is
shown in figure 4-b. The absorption characteristics of SnSiC are compared with that of SiC NCs in order to
appreciate the influence of Sn on the optical band-gap and
to further support the possibility of Sn incorporation in
the SiC NCs. SiC is a wide indirect band-gap
P-II-5-8
Figure 4:a) Attenuated total reflectance Fourier
transform infra-red spectra of Sn-SiC and SiC
nanocrystals; b) ultra-violet/visible absorption spectra
of Sn-SiC and SiC; Tauc plots of the respective
nanocrystals are displayed as an inset in the figure.
4. Conclusion
High quality Sn NCs were successfully synthesized by
using a simple, economical and potentially scalable
3
atmospheric microplasma setup. The NCs were narrowly
distributed over 1.1-2.4 nm with an average size of 1.6
nm. NCs charging within the plasma are considered
responsible for leading to well separated NCs. The
structure of NCs was found to be diamond cubic from
high resolution TEM (HR-TEM) and SAED patterns.
Furthermore, the same microplasma setup was extended
towards the incorporation of Sn in SiC NCs. A 7 %
reduction in the band-gap of SiC NCs, likely due to the
effect of Sn was also highlighted. While Sn NCs have
great potential to be used as anode materials for energy
storage applications, Sn alloying with other
semiconducting group IV NC can drastically contribute to
provide great opportunities in band-gap engineering of
environmentally
friendly
semiconducting
NCs.
Therefore, further work will be carried out to explore
interesting aspects offered by Sn in energy conversion and
storing devices.
5. Acknowledgments
This work was supported by the Marie Curie Initial
Training
Network
(RAPID-ITN),
EPSRC
(EP/K022237/1) and the Leverhulme International
Network (IN-2012-136). AH would like to acknowledge
the support of his personal Marie Curie Fellowship. SA
acknowledges the support of the Ulster University ViceChancellor’s Research studentship.
6. References
[1] Jesse M. Klostranec and Warren C. W. Chan, Adv.
Mater. 2006, 18, 1953.
[2] Arnim Henglein, Chem.Rev. 1989, 89, 1861.
[3] Benjamin F. P. McVey and Richard D. Tilley, Acc.
Chem. Res. 2014, 47, 3045.
[4] Pairot Moontragoon, Zoran Ikonic and Paul Harrison
Semicond. Sci. Technol. 2007, 22, 742
[5] Alexander A. Tonkikh, Nikolay D. Zakharov,
Alexandra A. Suvorova, Christian Eisenschmidt, Joerg
4
Schilling and Peter Werner, Cryst. Growth Des. 2014, 14,
1617
[6] Álvaro Caballero, Julián Morales, and Luis Sánchez,
Electrochemical and Solid-State Letters 2005, 8, A464.
[7] Yangang Wang, Bo Li, Chengli Zhang, Hong Tao,
Shifei Kang, Sheng Jiang, Xi Li, Journal of Power
Sources 2012, 219, 89.
[8] Linping Xu, Chunjoong Kim, Alpesh K. Shukla,
Angang Dong, Tracy M. Mattox, Delia J. Milliron, and
Jordi Cabana, Nano Lett. 2013, 13, 1800.
[9] Hyung Soon Im, Yong Jae Cho, Young Rok Lim,
Chan Su Jung, Dong Myung Jang, Jeunghee Park, Fazel
Shojaei, and Hong Seok Kang, ACS Nano 2013, 7,
11103.
[10] Guanhua Zhanga, Jian Zhu, Wei Zeng, Sucheng Hou,
Feilong Gong, Feng Li, Cheng Chao LI, Huigao Duan,
Nano Energy 2014, 9, 61.
[11] I. Arslan, T. J. V. Yates, N. D. Browning, P. A.
Midgley, Science 2005, 309, 2195
[12] Ryota Watanabe, Toshitaka Ishizaki, Bulletin of the
Chemical Society of Japan 2013, 86, 642
[13] Zhiguo Wang, Qiulei Su, Jianjian Shi, Huiqiu Deng,
G. Q. Yin, J. Guan, M. P. Wu, Y. L. Zhou, H. L. Lou and
Y. Q. Fu, ACS Appl. Mater. Interfaces 2014, 6, 6786.
[14] Tsidilkowski I M 1988 Gapless Semiconductors—A
New Class of Materials (Berlin: Akademie-Verlag)
[15] R.A. Soref and C.H. Perry, J. Appl. Phys. 1991, 69,
539.
[16] Davide Mariotti and R Mohan Sankaran, J. Phys. D:
Appl. Phys. 2010, 43, 323001.
[17] S. Askari1, I. Levchenko, K. Ostrikov, P. Maguire1
and D. Mariotti1, Applied Physics Letters 2014, 104,
163103.
[18] J. B. Casady and R. W. Johnson, Solid-State
Electronics 1996, 39, 1409.
[19] J. Tauc, Materials Research Bulletin 1968, 3, 37
P-II-5-8