Supplemental Material for
Crystalline Si nanoparticles below crystallization threshold: effects of
collisional heating in non-thermal atmospheric-pressure microplasmas
S. Askari1, I. Levchenko2,4, K. Ostrikov2,3,4, P. Maguire1 and D. Mariotti1
1
Nanotechnology & Integrated Bio-Engineering Centre (NIBEC), University of Ulster, BT37
0QB, UK
2
Plasma Nanoscience Centre Australia (PNCA), CSIRO Materials Science and Engineering,
P.O. Box 218, Lindfield, NSW 2070, Australia
3
School of Chemistry, Physics, and Mechanical Engineering, Queensland University of
Technology, Brisbane, QLD, 4000, Australia
4
School of Physics, The University of Sydney, Sydney, NSW, 2006, Australia
The chemical characterization of the silicon nanoparticles using energy-dispersive X-ray spectroscopy
(EDS) and X-ray photoelectron spectroscopy (XPS) is reported. EDS analysis was performed within a
JEOL JEM-2100F transmission electron microscope (TEM) and for XPS analysis an Axis Ultra DLD
spectrometer (Kratos Analytical, Japan) with monochromated Al Kα X-rays source.
For the TEM-EDS analysis, the Si nanoparticles (NPs) were synthesized with our plasma process and
collected directly in ethanol, using the same plasma synthesis conditions and collection procedure
presented in the main manuscript. The EDS profile and its corresponding TEM image from where the
EDS signal was collected is shown in figure S1, which confirms the presence of silicon and oxygen
while the carbon and copper peaks originate from the TEM grid and the weak chromium peak is
possibly from the stainless-steel holder of the grid.
(a)
(b)
Figure S1: (a) energy-dispersive X-ray spectrum (EDS) of the silicon nanoparticles and (b) its corresponding
transmission electron microscope image from where the EDS signal was collected.
A contribution to the oxygen peak may originate from the partial oxidation of the NPs; this may have
occurred during storage in ethanol, due to absorbed water vapour from air, and/or from air exposure
during preparation of the samples on the TEM grids. The EDS analysis is here useful to confirm that
silicon and oxygen are the only possible constituents of the nanoparticles; however, the quantification
of the EDS spectra would not aid the identification of the exact Si/O composition as the oxygen peak
may include contributions from the different components of the TEM grid.
XPS analysis is therefore used for further investigation of the chemical composition. The Si 2p XPS
spectra of the Si NPs powder is shown in figure S2. The detailed de-convolution of the profile is
performed by fitting Gaussians curves corresponding to metallic silicon Si0 at 99.4 eV and peaks
corresponding to different oxidation states (Si1+, Si2+, Si3+), which are equally spaced from the binding
energy of Si0 to that of Si4+ (103.4 eV) [1-3].
Figure S2: X-ray photoelectron spectra of the Si 2p peak of silicon nanoparticles.
The XPS spectrum shows that a large component of the sample includes Si-Si bonds confirming the
presence of non-oxidized silicon. This is attributed to the core of the Si NPs supported by the TEM
analysis in the main manuscript. Partial oxidation is also present where the different oxidation states
originate from an interface between a pure silicon component and an oxide, possibly located at the
interface between the NP core and its oxide shell; the amorphous oxide shell may be visible in figure
2c of the main manuscript, however, at the moment, this cannot be located with clarity in the TEM
images. As already mentioned, a degree of oxidation is expected as the samples are exposed to air
when transferred from the synthesis set-up to the XPS chamber (< 24 hours).
The results from TEM, TEM-EDS and XPS analysis confirm that our process produce crystalline Si
NPs. A degree of oxidation may be present and is in agreement with other reports on porous and other
nanostructured silicon (e.g. [1-3]). Further work will be carried out to determine the thickness,
conformation of the oxide.
The photoluminecence (PL) at three excitation wavelengths and absorption of the nanoparticles
dispersed in ethanol is shown in figure S3a. Furthermore, the size distribution of the Si NPs
determined from TEM images is shown in figure S3b, which shows that most NPs have a diameter
within the range 1.75-2.25 nm.
Figure S3: (a) photoluminecence and the ultra-violet visible absorption spectra of Si nanoparticles (NPs)
dispersed in ethanol with three difference excitation wavelength (the inset is a photo of the Si NPs in ethanol
under excitation); (b) size distribution (diameter) of the Si NPs based on transmission electron images.
The shift of the PL is likely due to the size distribution whereby a longer excitation wavelength would
be able to excite only larger particles and for this reason the PL peak would be seen to red-shift with
increasing excitation wavelength. However, the interpretation of PL from quantum confined Si NPs is
still subject of debate and can originate from indirect transitions (where the momentum conservation
is relaxed), direct transitions and surface/interface recombination (see e.g. [4-5]). At this time we are
unable to comment any further on the PL origin as dedicated studies on the optical properties would
be required; this analysis would be well beyond the scope of this work and will be reported elsewhere
in the future.
References
1. A. Thøgersen, J H Selj, E S Marstein, Journal of the Electrochemical Society, 159, D276, 2012
2. D. Q. Yang, J. N. Gillet, M. Meunier, and E. Sacher, J. Appl. Phys, 97, 024303, 2005
3. W. L. Zhang, S. Zhang, M. Yang, and T. P. Chen, J. Phys. Chem. C, 114, 2414, 2010.
4. F. A. Reboredo and G. Galli, J. Phys. Chem. B, 109, 1072, 2005.
5. W. D. A. M. de Boer, D. Timmerman, K. Dohnalova, I. N. Yassievich, H. Zhang, W. J. Buma and T.
Gregorkiewicz, Nat. Nanotechnol., 5, 878, 2010.