Full text of DOI:10.1007/BF02901158

Fröhlich interaction, the calculated result matches the ob-
served spectra well[3]. Thus, the shift in the LO mode can
be understood and indicates that the LO and the interface
mode will be stronger than the TO mode.
Raman spectra of SiC nano-
rods with different excitation
wavelengths
YAN Yan1, HUANG Fumin1, ZHANG Shulin1,
ZHU Bangfen3, SHANG Eryi2 & FAN Shoushan3
The SiC-NR sample used in our experiments were
prepared by the method of Dai et al.[1], where carbon
nanotube (CNT) was allowed to react with SiO vapor at
high temperature. Our SiC-NR samples contained solid
SiC covered by amorphous SiO2 and/or SiO. It is clear in
the transmission electron microscopy images of the sam-
ple that most nano-rods contain a high density of defects.
The diameters of these nanorods are about 10 nm on the
average, and their lengths can be up to more than 10 um[4].
X-ray diffraction spectra indicate that the crystalline
structure of the sample is predominantly that of the
zincblende type (E-SiC). While the crystalline direction in
SiC NRs is not unique, the predominant directions are
found to be [111], [220], and [311]. Renishaw model 1000
Raman microscope was used for Raman spectral mea-
surements. Excitation wavelengths used were 633, 514
and 782 nm. All Raman measurements were done at room
temperature with back-scattering geometry.
1. Department of Physics, Peking University, Beijing 100871, China;
2. Institute of Semiconductor, Chinese Academy of Sciences, Beijing
100083, China;
3. Department of Physics, Tsinghua University, Beijing 100084, China
Correspondence should be addressed to Zhang Shulin (e-mail: slzhang@
pku.edu.cn)
Abstract
With increasing excitation wavelength from 514
to 782 nm, a significant difference in the Raman spectra of
SiC nanorods was observed as compared to bulk material.
The intensity ratio of the LO mode to that of the IF mode
increases with the excitation wavelength increasing. This has
been identified as resonant Raman scattering caused by
Fröhlich interaction.
Keywords: SiC NR, Raman spectroscopy, Fröhlich interaction.
Recently, we found that when the excitation wave-
length changed, the Raman spectral features of SiC NR
changed a lot. Typical results are shown in fig. 1. In sharp
contrast to the published spectrum at 633 nm excitation,
there are almost no LO and IF peaks in the spectrum ex-
cited by 514 nm. With the excitation wavelength increas-
ing, the Raman peak intensities in the LO and IF region
increase. At 782 nm excitation, the intensity of LO mode
even surpasses that of the TO mode (fig. 1). To better un-
derstand this variation, the spectra in fig. 1 were fit to
Gaussian lineshapes. The result of the fit for 633 nm exci-
tation is shown as dashed lines in fig. 2. The peak at ca.
770 cmꢀ1 was identified as the second order peak of rem-
nant SiO2 in the sample[5], while the peaks at ca. 793 and
928 cmꢀ1 were assigned to the TO and LO mode respec-
tively. The other peaks at ca. 838 and 882 cmꢀ1 may be
due to the IF mode scattering from different kinds of in-
terfaces in the sample.
Recently, various methods have produced many
novel one-dimensional (1D) materials[1]. Understanding
the physical properties of these new materials including
their phonon behavior is both necessary and crucial for
innovative technology applications. It is also of funda-
mental interest. As a high temperature and radiation-tol-
erant material, SiC has proved to be useful in the micro-
electronic and opto-electronic industry. Now SiC attracts
further interest as SiC nanorods (NRs) have been synthe-
sized successfully.
The bulk E-SiC is of the zincblende structure. It has
two optical phonon modes at the *-point of the Brillouin
zone, a TO mode at 796 cmꢀ1 and an LO mode at 972
cmꢀ1. In contrast, the TO and LO phonon modes of SiC
NR are at ca. 791 and 924 cmꢀ1, respectively (with 633
nm excitation). The frequency shift of the TO mode of 5
cmꢀ1 can be adequately interpreted by the size confine-
ment effect. However, the shift of the LO mode of up to
45 cmꢀ1 and the weak peak at 856 cmꢀ1 due to the defects
cannot be easily explained. Note that the presence of
abundant defects in the SiC NR sample will break the
translation symmetry and relax wave vector momentum
conservation. Consequently, all phonon modes can con-
tribute to the Raman scattering process in principle. Thus,
the observed peaks in the first-order Raman spectrum may
be attributed to the peak of the effective density of phonon
states rather than to the phonon peak as in conventional
interpretation. To verify the above analysis, the first order
Stokes Raman scattering intensity can be calculated by[2]
CbDE ,JG (1/Z)[1ꢁ n(Z)]Db (Z)
.
IDE ,JG (Z)
¦
b
Fig. 1. Observed Raman spectra of SiC NRs excited at different wave-
lengths.
Considering that LO phonons are enhanced by the
Chinese Science Bulletin Vol. 46 No. 22 November 2001
1865

NOTES
Expression (1) shows that the scattering intensity increase
of LO modes as excitation energy approaches the gap en-
ergy much faster than that of TO modes. For polar materi-
als, the densities of the state of the LO mode and the in-
terface mode are enhanced by Fröhlich interaction, and
the above analysis can explain the strong dependence of
the LO and the interface mode with exciting wavelength.
In summary, we have observed a novel resonance
Raman scattering in polar semiconductor SiC one-dimen-
sional material. Furthermore, for polar nano-scale semi-
conductor materials with abundant defects, Fröhlich en-
hancement and the presence of defect play a more signifi-
cant role than the quantum confinement effect in optical
phonon Raman scattering.
Fig. 2. Fitted Raman spectra of SiC NRs excited at 633 nm.
Raman scattering theory for bulk material tells us
that the scattering mechanism of the LO and TO mode is
different; scattering is induced by Fröhlich interaction for
the LO mode, while the TO mode is due to deformation in
the potential. The electron-optical phonon interaction
Hamiltonian can be given by[6]
Acknowledgements The authors would like to thank Prof. K. T. Yue
for the helpful discussion. This work was supported by he National
Natural Science Foundation of China (Grant Nos. 69476037, 17774009
and 69876001), and the State Commission of Science and Technology.
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Heꢀph 4ʌ e2K ZLO (Hfꢀ1 ꢀH0ꢀ1
)
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ꢁ
g
g
,
]
exp(iq r)Yn (q)VnE (z) [anE ꢁ anE
¦¦
q,n E r
where
is the LO phonon frequency of bulk SiC,
ZLO
ꢁ
ꢀ
,
are phonon creation, annihilation operators,
anE anE
respectively, VnEꢂ(z) is the potential function. Once the
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2
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c
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Information on the intensity of Raman peaks is given by
Heꢀph
.
For Raman scattering due to deformation and Fröh-
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scattering cross section Vꢂ on gap energy Eg and incident
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ꢀ1 , for deformation interaction;
V v (Eg ꢀ !Z)
(Received June 20, 2001)
ꢀ3 , for Fröhlich interaction.
(1)
V v (Eg ꢀ !Z)
1866
Chinese Science Bulletin Vol. 46 No. 22 November 2001