SUPPORTING INFORMATION
Thermochemically evolved nanoplatelets of bismuth
Selenide with enhanced thermoelectric figure of
merit
Zulfiqar Ali,1 Sajid Butt,2 Chuanbao Cao,1* Faheem K. Butt,1 Muhammad Tahir,1 M. Tanveer,1
Imran Aslam,1 Muhammad Rizwan,1 Faryal Idrees,1 Syed Khalid,1.
1
Research Centre of Materials Science, School of Materials Science and Engineering, Beijing
Institute of Technology, Beijing100081, People’s Republic of China.
2
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and
Engineering, Tsinghua University, Beijing 100084, People’s Republic of China.
*Corresponding Author
Email: cbcao@bit.edu.cn (Chuanbao Cao)
Tel: +86 10 68913792. Fax: +86 10 68912001
XRD and EDS Results
Figure S1(a) shows the XRD patterns of Bi2Se3 synthesized in a furnace tube using Be and Se
powders as precursors which depicts that all the characteristic peaks correspond to pure
rhombohedral Bi2Se3 and the Millar indices of every peak are indexed. The main peaks located at
2θ = 29.37o, 40.26o, and 43.68o are corresponding to (015), (1010) and (110) planes respectively,
have the highest intensity. The lattice parameters calculated for Bi2Se3 are a=4.1368A° and
c=28.6342A°, which correspond to the standard values of JCPDS card No.33-0214 for Bi2Se3.
Inset of Fig. S1(a) represent the XRD pattern of pure phase bismuth selenide with different
morphologies controlled at different synthetic conditions. It can be seen from Fig. S1(b) that the
synthesis temperature has a profound effect on the purity of the product.
Figure S1(a) Millar Indices marked XRD spectrum of as synthesized product and inset shows
the XRD of all samples (b) XRD spectrum of the samples prepared at different temperatures
showing oxide peaks (c) EDS analysis of the sample prepared at 700oC showing oxide peak (d)
EDS analysis of the sample prepared at 650oC showing pure phase bismuth selenide (e) EDS
analysis of the sample prepared at 600oC showing different ratios of bismuth and selenium.
Two solid solutions of bismuth selenide are present near melting point which is 705 oC. One is
Se-rich and other is Bi-rich, Eutectic reaction is supposed to exist in the Se-rich part of the
Bi2Se3 system1 while the Bi-rich part of the Bi2Se3 consists of a sequence of peritectic reactions.
Purity of the product was further investigated by EDS analysis. Fig S1(c) represents the EDS
investigation for the products synthesized at 700oC. Oxide peak can clearly be observed
suggesting that as the temperature increases above 700oC, oxide starts to form in the melt while
Fig. S1(d) shows pure phase bismuth selenide at temperatures lower than 700 oC. Fig. S1(e)
shows that the stoichiometric ratio of Se to Bi decreases when the reaction time increases but
below 700 0C. Bismuth and Selenium are present in 71.20:28.80 ratios while in 87.69:12.31
ratios for longer reaction times.
Photoelectron Spectroscopy (XPS) Results
Figure S2 shows the photoelectron spectra (XPS) of selected sample with 50nm thickness of
Bi2Se3 platelets. From the full scale survey spectrum, it is evident that all the spectral features
observed points towards the pure phase bismuth selenide. The comparison of the XPS obtained
from products synthesized at 700oC and at 600oC is presented in Fig. S2(b). The XPS survey
suggests that when temperature is 700oC, an extra peak near the overlap peaks at the binding
energy of 160 eV can be seen, this extra peak correspondents to Bi 4f7/2 line related to Bi-O
bonds. The presence of this phase was also detected by XRD which confirmed that at 700 oC, the
formation of Bi2O3 starts.
Fig. S2(a) XPS analysis of as synthesized pure phase Bi2Se3 (b) XPS analyses of the product at different
temperatures (c) Photoelectron spectrum of Bi 5d in Bi2Se3 (d) Photoelectron spectrum of Se 3d in Bi2Se3.
The Bi 5d doublet are resolved and presented in Fig. 3(c). While Fig. 3(d) shows the Se 3d
doublets resolved in the as synthesized pure phase Bi2Se3.
Table S1 Binding Energies of constituent elements in as synthesized Bi2Se3.
Bi 4f7/2
Bi 4d5
Bi 4d3
Se 3d5
Se 3d3
[eV]
[eV]
[eV]
[eV]
[eV]
[eV]
24.8
157.9
-
-
53.7
-
3
25.5
157.6
-
-
53.3
-
2
Bi 5d5/2
Reference
27.4
157.6
440.7
464.2
53.3
54.0
4
24.8
157.9
-
-
53.35
54.1
5
24.6
157.7
-
-
53.5
54.2
6
24.8
157.7
441.2
466.2
53.2
54.1
Our study
Table S1 represents the core levels measured from bismuth selenide together with the parameters
reported in the literature. The corresponding binding energy values for elemental Bi and Se given
in literature3 are 24.8, 157.3 and 55.7 for Bi 5d5/2, Bi 4f7/2 and Se 3d5/2 respectively.
Controlling Parameters
Table S2. Controlling Parameters for Bi2Se3 morphology.
Morphology
Reaction
Temperature
Ramp Rate
Reaction Time
Ar Flow Rate
Thickness
[oC/min]
[Hrs]
[SCCM]
[nm]
500-650
600
600
10
2
2
2
2
4
50
50
50
200-500
1000
100
650
2
2
50
500
650
2
4
50
50
5
2
50
50-1000
[oC]
Nanoplates
Nanosheets
Nanosheets
Self
Assembled
Flower like
sheets
Self
Assembled
Flower like
sheets
Bi2O3 Sheets
>700
Raman Spectroscopy Results
Fig. S3(a) Raman Spectroscopy of both the selected samples showing different modes of
vibrations (colour online) (b) Analysis of A11g vibrational mode in both the samples (c) Analysis
of E2g vibrational mode in both the samples (d) Analysis of A21g vibrational mode in both the
samples.
Comparison of the full-width-at-half-maximum (FWHM) of Eg2 mode gives important
information about the thickness of sheets. The analysis carried out for platelets under discussion
suggests that the two samples have approximately 100 and 50nm thickness which was also
predicted by SEM results. Fig. S3(a) shows the Raman spectra for the two selected samples with
known thickness. In both the samples, three vibrational modes can clearly be seen at 71.23 cm-1,
131.05 cm-1 and at 173.45 cm-1, respectively. These three peaks correspond to pure phase
bismuth selenide and agree well with the reported results. Fig. S3(b) shows Raman shift in A11g
vibrational mode. It is evident from the figure that for the difference in thickness of 50 nm, the
peak position shifts about 0.9 cm-1 and the intensity is increased in the thin sample by an amount
of 183.76 (measured in arbitrary units). Similarly, Raman shifts and intensity differences are
calculated for E2g and A21g vibrational modes for the difference in 50 nm thickness sheets. From
the figure it can be seen that E2g vibrational mode is most sensitive to the thickness, for a 50nm
thickness difference, there is a Raman shift of about 2.17 cm-1 it also shows the maximum peak
broadening of about 10 cm-1, while this mode shows the lowest intensity difference. The
broadening of peaks and red shift of the in-plane vibrational mode E2g suggest that layer to layer
stacking has profound effect on the Bi-Se atomic bonds and decrease in phonon life time, which
are also confirmed by our XPS results. The wave interference is very important in nanodevices to
understand the thermal transport. The wavelengths of phonons are similar to the length scale of
the microstructure. In most solids all phonon states in Brillouin zone are involved in the transport
at room temperature. Their wavelengths span the range from atomic dimension to the size of the
sample. Most of the heat is carried away by the phonons with wavelengths of a few nanometers.
On this scale wave interference becomes important when mean free path is not large enough for
phonon band gaps to occur. The lattice thermal conductivity data in figure S3 below indicates the
anharmonic phonon scattering over the reported temperature range which indicate the higher
quality material7 as predicted by XPS results. This red shift and broadening in the peaks of E2g
mode is purely a function of thickness while the difference in intensities can be attributed to the
quality of product. Therefore, we can conclude that in-plane vibrational mode E2g is more
sensitive to thickness as compared to the out of plane modes even in the ex-quintuple layer
regime.
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