Optical spectroscopy of silicon nanowires
Jifa Qi
a,
*
, John M. White
a
, Angela M. Belcher
a
, Yasuaki Masumoto
b
a
Department of Chemistry and Biochemistry, University of Texas, Welch 4.212, Austin, TX 78712, USA
b
Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
Received 16 December 2002; in final form 13 March 2003
Abstract
Silicon nanowires (SiNWs) were prepared by laser ablation at high temperature and studied by electron microscopy
and optical spectroscopy. As-synthesized SiNWs are found orderly aligned on the silica substrates, exhibiting uniform
shape with a silicon crystalline core and an amorphous silicon oxide sheath. Asymmetrically broadened Raman spectral
peaks downshifted from 520 cm
À1
were observed, which related to the confinement effects of optical phonon by
nanowire boundaries. The SiNWs showed strong photoluminescence (PL) bands peaked at 455 and 525 nm, which
quenches rapidly with an increase in temperature and may arise from the defects surrounding the silicon nanowire
crystalline core.
Ó 2003 Elsevier Science B.V. All rights reserved.
Si nanowires (SiNWs) are expected to exhibit
potentially useful electrical, optical, mechanical,
and chemical properties due to their small di-
mensions, unique shapes, and high surface-to-
volume ratio. The recent progress in large-scale

Corresponding author. Present address: Department of
Material Science and Engineering, Massachusetts Institute of
Technology, 77 Massachusetts Ave., #16-244, Cambridge, MA
02139, USA. Fax: +6173243300.
E-mail address: (J. Qi).
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0009-2614(03)00504-9
passed through the quartz tube at a flow rate of
50–100 standard cubic centimeters per second
(sccm). A pulsed XeCl excimer laser (308 nm, pulse
repetition 10 Hz, energy 170 mJ; Lambda Physik
product) was used to ablate the target for 3 h while
the furnace temperature was kept at 1200 °C. The
product was collected from the silica tube wall. A
Hitachi H9000 and a JEOL 2010 transmission
electron microscopes (TEM) working at 200 kV
were used to characterize the products.
Fig. 1 shows the typical electron microscopic
image of the morphology of the SiNWs. It was
observed that the product exemplified high purity
with a uniform diameter 20 nm and consisted of
most of the SiNWs aligning on the substrate. Fig.
2 shows a high resolution TEM (HRTEM) image
of a single SiNW with a diameter of about 18 nm.
The (1 1 1) lattice fringes with the interplanar
spacing of 0.31 nm and the corresponding selected
area electron diffraction (SAED) patterns show the
SiNW consisted of a crystalline Si structure. Ad-
ditionally, there is a thin amorphous silicon oxide
layer (about 3 nm) sheathing the crystalline core of

2
.
764 J. Qi et al. / Chemical Physics Letters 372 (2003) 763–766
intense Raman line at 520 cm
À1
with the full width
at half maximum (FWHM) of 4:7cm
À1
was wit-
nessed in the Raman spectrum of crystal Si. This
peak corresponds to the degenerate zone-center
optical phonon mode of crystal Si. All SiNW
samples exhibit similar Raman spectral peaks red-
shifted from 520 cm
À1
and a small shoulder at
495 cm
À1
. The main peak near 520 cm
À1
corre-
sponds to the first-order optical phonon of crys-
talline Si. The small broad peak at 495 cm
À1
was
attributed to the amorphous silicon that covers
SiNWs or distributed on the silica substrate, which
has a Raman structure between 400 and 550 cm
À1
peaked at 480 cm

shows the best-fit result, and the average crystal
size of nanowires D ¼ 11:3 nm was obtained, as
shown in Fig. 3. The good agreement in spectral
features between experimental and calculated Ra-
man spectra indicates that the identification of the
Raman peak of SiNW is correct. However, the
diameters determined by Raman scattering mea-
surements were smaller than that obtained from
SEM and TEM observation. The reasons for this
disparity are considered below. First, only the
crystallite contributes to the main Raman scat-
tering peaks, our nanowires are capped by the
amorphous oxide layer and amorphous silicon,
their contributions to the Raman spectra were not
calculated. Second, the existence of defects and
stresses in SiNWs can have a profound influence
on the Raman spectra of SiNWs.
The PL measurements have been performed by
an experimental setup consisting of the excitation
source of a He–Cd laser (325 nm) and a 27.5 cm
monochromator equipped with a liquid nitrogen
cooled CCD detector. In order to investigate the
PL as a function of temperature, samples were
placed in a temperature-variable cryostat. The PL
spectra were measured at temperatures ranging
from 10 to 300 K. Fig. 4 shows the PL spectra of
SiNWs at different temperatures. Two strong
emission bands in the green and blue regions re-
vealed peaking at 455 and 525 nm at low temper-
ature, respectively. The band that peaked at 455

dependence of the luminescence intensity can be
simply written by [10]
IðT Þ¼
I
0
1 þ C
A
expðÀE
A
=kT ÞþC
B
expðÀE
B
=kT Þ
;
ð2Þ
E
A
and E
B
are thermal activation energies of cen-
ters A and B, respectively, while C
A
and C
B
are
temperature-independent factors. The fit result by
using Eq. (2) presented by the solid line in the inset
of Fig. 4 shows a good coincidence with the ex-
periment results. The best-fit parameters are

downshifted and broadened Raman spectral peak
was observed, which is related to the confinement
effects of optical phonons by the nanowire
boundaries. SiNWs emit green and blue light un-
der ultraviolet photoexcitation. The green and
blue bands are related to the radiative recombi-
nation of the defect centers in the outer oxide layer
of the SiNWs. The luminescence quenches rapidly
with an increase of temperature.
Acknowledgements
The authors would like to thank the Research
Center for Advanced Carbon Materials, AIST, for
use of the micro-Raman spectrometer instrument.
References
[1] M. Morales, C.M. Lieber, Science 279 (1998) 208.
[2] D.P. Yu, C.S. Lee, I. Bello, X.S. Sun, Y.H. Tang, G.W.
Zhou, Z.G. Bai, Z. Zhang, S.Q. Feng, Solid State
Commun. 105 (1998) 403.
[3] Y.F. Zhang, Y.H. Zhang, N. Wang, D.P. Yu, C.S. Lee,
I. Bello, S.T. Lee, Appl. Phys. Lett. 72 (1998) 1835.
[4] J. Qi, Y. Masumoto, Mater. Res. Bull. 36 (2001) 1407.
[5] D.P. Yu, Q.L. Hang, Y. Ding, H.Z. Zhang, Z.G. Bai, J.J.
Wang, Y.H. Zou, W. Qian, G.C. Xiong, S.Q. Feng, Appl.
Phys. Lett. 73 (1998) 3076.
[6] Z.G. Bai, D.P. Yu, J.J. Wang, Y.H. Zou, W. Qian, J.S. Fu,
S.Q. Feng, J. Xu, L.P. You, Mater. Sci. Eng. B 72 (2000)
117.
[7] Z. Iqbal, S. Vep

rrek, J. Phys. C 15 (1982) 377.