2 3 topological insulator at the 2D-limit: Role of halide-doping on … · 2018-06-08 · S1 Electronic supplementary information (ESI) Bi 2 Se 3 topological insulator at the 2D-limit:

S1

Electronic supplementary information (ESI)

Bi2Se3 topological insulator at the 2D-limit: Role of halide-doping on Dirac point

Salma Khatun, Hrishikesh Bhunia, and Amlan J. Pal*

Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur,

Kolkata 700032, India

E-mail: [email protected].

Characterization of nanoplates

The synthesized nanoplates were characterized with optical absorption spectroscopy, X-ray

diffraction (XRD) studies, transmission electron microscopy (TEM) and high-resolution TEM

(HR-TEM) images along with energy-dispersive X-ray (EDX) analyses, and X-ray photoelectron

spectroscopy (XPS). The measurements were performed with a Cary 5000 UV-Vis-NIR

spectrophotometer, Bruker D8 Advanced X-ray Powder Diffractometer (Cu K radiation, =

1.54 Å), and JEM 2100F Jeol TEM, and an XPS instrument (Omicron: Serial no. 0571),

respectively.

Optical absorbance and band gap

At first, pristine and doped sample are characterized by optical absorption spectroscopy. It also

allowed us to look for any influence of dopants on the optical gap of Bi2Se3 nanoplates. To do so,

we have recorded optical absorption spectra of their thin-films (Fig. S1a). The spectrum of

pristine nanoflakes resembled the reported results.S1,S2

In Fig. S1b, we show a plot of (Ahν)2

versus energy spectrum (Tauc plot) to obtain the optical gap of the materials, where A represents

absorbance, h the Planck’s constant, and the frequency. The optical gap can be derived by

drawing a tangent in the linear region of the plot. A gap of around 0.6 eV matched well with the

reported resultsS1,S2

halide doping did not affect the band gap of the materials.

Electronic Supplementary Material (ESI) for Physical Chemistry Chemical Physics.This journal is © the Owner Societies 2018

S2

Fig. S1 (a) Normalized optical absorption spectra and (b) Tauc plot of Bi2Se3 and X@Bi2Se3 nanoflakes in their

thin-film form, where X = Cl, Br and I.

X-ray diffraction (XRD) studies

For structural studies, we have recorded XRD patterns of the materials (Fig. S2). The patterns

matched well the JCPDS Card No. 33-0214, which represent Bi2Se3 in their rhombohedral

geometry. In the patterns of doped-Bi2Se3, neither noticeable shift of peaks nor extra

peaks/patterns appeared. The results inferred successful substitution of dopant-ions in Bi2Se3

without formation of any additional subsidiary phases.

Fig. S2 X-ray diffraction patterns of Bi2Se3 and X@Bi2Se3 nanoplates, where X = Cl, Br and I, Lattice planes

matched well with JCPDS Card Number #33-0214.

10 20 30 40 50 60 70 80

I@Bi2Se

3

Br@Bi2Se

3

In

ten

sity (

a.u

.)

Cl@Bi2Se

3

Bi2Se

3

2 degree)

500 1000 1500 2000 2500

Bi2Se

3

Cl@Bi2Se

3

Br@Bi2Se

3

I@Bi2Se

3

Ab

so

rba

nce

(a

.u.)

Wavelength (nm)

(a)

0.4 0.6 0.8 1.0

Bi2Se

3

Cl@Bi2Se

3

Br@Bi2Se

3

I@Bi2Se

3

(Ah

)2 (

a.u

.)

Energy (eV)

(b)

S3

(a) (c)

(d)

(b)

Transmission electron microscopy (TEM)

Structural and elemental compositions are also studied through TEM and EDS analyses. TEM

and HR-TEM images, diffraction pattern, and EDS spectrum have been clubbed in Figs. S3-S6

for Bi2Se3, Cl@Bi2Se3, Br@Bi2Se3 and I@Bi2Se3, respectively. Along with each HR-TEM

image, we have added a simulated atomic image of the nanocrystals obtained through analysis of

a TEM image with Gatan Microscopy Suite (GMS). Hexagonal symmetry of Bi2Se3 and also of

doped-Bi2Se3 nanocrystals could be viewed clearly by observing the diffraction pattern.

Interplanar spacing of 2.084 Å corresponded well with the (110) planes of Bi2Se3. This value

matched well also with the JCPDS card, which assigned the planes to have a spacing of 2.070 Å.

Existence of dopants could be established in EDS analyses; the dopant-content matched

reasonably well with the value that was envisaged during the colloidal synthesis process. The

atomic percentage of different elements in pristine and doped samples is given in Table S1.

Fig. S3 (a) TEM images, (b) HR-TEM image (inset shows the simulated atomic pattern), (c) diffraction patterns,

and (d) EDS spectrum of Bi2Se3 nanoplates.

S4

(a) (b) (c)

(d)

(a) (b) (c)

(d)

Fig. S4 (a) TEM images, (b) HR-TEM image (inset shows the simulated atomic pattern), (c) diffraction patterns,

and (d) EDS spectrum of Cl@Bi2Se3 nanoplates.

Fig. S5 (a) TEM images, (b) HR-TEM image (inset shows the simulated atomic pattern), (c) diffraction patterns, and

(d) EDS spectrum of Br@Bi2Se3 nanoplates.

S5

(c)(b)(a)

(d)

Fig. S6 (a) TEM images, (b) HR-TEM image (inset shows the simulated atomic pattern), (c) diffraction patterns, and

(d) EDS spectrum of I@Bi2Se3 nanoplates.

Table S1. Atomic percentages of bismuth, selenium and dopants from EDS analysis

Material Atomic Percentage

Bi Se Dopant

Bi2Se3 46.5 53.5 –

Cl@Bi2Se3 46.0 49.1 4.9

Br@Bi2Se3 45.0 50.0 5.0

I@Bi2Se3 45.8 49.7 4.5

S6

XPS analysis

To find out information on the valence state of the constituent elements and also doped halide,

we carried out XPS of one doped Bi2Se3 nanoplates. Fig. S7 shows the obtained results for

Cl@Bi2Se3 that was corrected by considering the peak of C1s to appear at 284.5 eV. Non-

existence of peaks due to elements other than Bi, Se, Cl, O and C concluded a high purity of our

sample. Peaks for Bi 4f and Se 3p were observed to overlap near 160 eV. High-resolution spectra

of Bi 4d and Bi 4f have been presented in Figs. S7(b) and (c), respectively. Bi 4d5/2 and 4d3/2

peaks at 440.1 and 464.2 eV, respectively and Bi 4f7/2 and 4f5/2 peaks at 157.1 and 162.5 eV,

respectively were observed in the high-resolution spectrum. Cl 2s and 2p peaks could be

observed at 269.0 eV and 198.9 eV, respectively. Similarly, the Se 3d state appeared at 53.6 eV.

All the energy values agreed well with the NIST X-ray Photoelectron Spectroscopy Database.

S7

Fig. S7 (a) Survey XPS spectrum of Cl@Bi2Se3; (b)-(e) represent high-resolution spectra showing Bi 4d, Bi 4f, Cl

2p, and Cl 2s states, respectively.

0 200 400 600 800S

e 3

S

Bi 4

p3

/2

O 1

S

Bi 4

d3

/2B

i 4

d5

/2

C 1

S

LM

M-I

IS

e 3

pB

i 4

f L

MM

-I

Se

3d

Inte

nsity (

a.u

.)

Binding Energy (eV)

Bi 5

d

(a)

430 440 450 460 470

Bi 4

d3/2

Inte

nsity (

a.u

.)

Binding Energy (eV)

Bi 4

d5/2

(b)

150 155 160 165 170

Bi 4

f 5/2

Inte

nsity (

a.u

.)

Binding Energy (eV)

Bi 4

f 7/2

(c)

196 198 200 202 204

(d)

Inte

nsity (

a.u

.)

Binding Energy (eV)

Cl 2p

266 268 270 272 274

(e)

Inte

nsity (

a.u

.)

Binding Energy (eV)

Cl 2s

S8

References

S1 T. E. Manjulavalli, T. Balasubramanian and D. Nataraj, Chalcogenide Lett., 2008, 5, 297-302.

S2 Y. Sharma, P. Srivastava, A. Dashora, L. Vadkhiya, M. K. Bhayani, R. Jain, A. R. Jani and B. L.

Ahuja, Solid State Sciences, 2012, 14, 241-249.