Si quantum dots with a high absorption coefficient:
analysis based on both intensive and extensive variables
Takumi Kitasako1 and Ken-ichi Saitow1,2,a)
1
Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1
Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan
2
Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University,
1-3-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan
Supplemental materials
Experimental details
The absorption spectra in UV-Visible region were recorded using a commercial
spectrophotometer (V-660, JASCO). The absorption spectra of Si-QDs dispersed in 2-propanol
(Nacalai Tesque, Inc.) were measured using a quartz cell with the optical path length of 1 cm. In
the spectral measurement, we used 2-propanol solvent in a quartz cell as a reference. To consider
carefully the value of absorbance, the blank absorption spectrum was checked using 2-propanol
solvent against the air as a reference (Figure S1); e.g. Abs = 0.1 at 225 nm 0.02 at 237 nm. As
shown in the data, the solvent absorbance is lower than 0.01 in the current wavelength range
(240-800 nm). Thus, the obtained data corresponds to data measured at a solvent-absorption free
condition. From the point of view of instrument, the spectrophotometer has been ensured to have
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a linearity up to Abs. = 6 at the longer wavelength than 220 nm. Thus, it is safely mentioned that
the measured absorbance (Abs=0.015-4.0) in the present study, ranging from 240 nm to 800 nm,
is an accurate value. Under these conditions, the absorption spectra of Si-QD dispersed in
2-propanol solvent were measured four times. The averaged spectrum was used to obtain the
extinction coefficient and absorption coefficient. To evaluate the extinction coefficient and
absorption coefficient, the error bars were calculated using the progression of experimental errors.
Fourier transform IR (FTIR) spectrum of Si-QDs was measured using a commercial apparatus
(FT/IR-4200, JASCO), based on an attenuated total reflectance method. The sample and
reference IR spectra were measured by dropping droplets of 2-propanol dispersion of Si-QDs and
2-propanol solvent on the ATR prism made of ZnSe (ATR PRO450-S, JASCO), respectively.
Using the sample and reference spectra, the IR absorption spectrum was obtained, based on the
Lambert-Beer law.
The size distribution of Si-QDs in the 2-propanol was measured using a commercial
DLS apparatus (Zetasizer Nano, Malvern Instruments Ltd.) at the laser wavelength of 532 nm.
Using five different DLS apparatuses produced from different companies, it was ensued that the
apparatus used in the current study gives the best profile of time correlation function, measured at
the wavelength of 532 nm, to obtain a certain size distribution function and the good repeatability
of data. Namely, to ensure the repeatability of the measurements and analysis, both the
time-correlation and size distribution functions were measured 13 times and good repeatability
was ensured.
The size distribution of Si-QDs at the dried condition was investigated by measuring
transmission electron microscope (TEM) images. The TEM images were captured with TEM
(JEM-2011, JEOL) at the acceleration voltage of 200 kV. The TEM sample was were prepared by
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dropping a 2-propanol dispersion of Si-QDs on TEM grids (High Resolution Carbon Grid STEM
Cu100P, Okenshoji Co., Ltd.). Using the same TEM apparatus and sample, the elemental analysis
of Si-QDs was conducted by measuring the spectrum of silicon with an energy dispersive x-ray
spectrometer (JED-2300T, JEOL) and the particle made of silicon was ensured. High resolution
data is shown in Fig 1(c), and larger area data is displayed in Figure S2.
The concentration of Si-QDs solution was evaluated by measuring silicon atom
concentration with an inductively coupled plasma optical emission spectroscopy (ICP-OES). The
data was collected by measuring the silicon luminescence at the wavelength of 251.611 nm with a
commercial IOP-OES (ICP Expert Ⅱ, Agilent Technologies). Standard solution of silicon
(Kanto Chemical Co., Inc.) was prepared to obtain a standard curve, giving the linear relationship
between the emission intensity to the concentration of Si atom. The solution of Si-QDs dispersed
in 2-propanol was measured using the measured standard curve. The 2-propanol was also used as
a fluent solvent for the ICP-OES measurement. To ensure the repeatability of the measurements
and analysis, the ICP-OES measurements were conducted 3 times at every measurement.
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2.0
Absorbance
1.5
1.0
0.5
0.0
200
300
400
500
600
700
800
Wavelength (nm)
Figure S1 Absorption spectrum of 1-propanol for evaluating the solvent absorption
5 nm
Figure S2. TEM image of Si-QD. The data corresponds to the large area image of Fig. 1(c).
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