RAMAN FREQUENCY VARIATION WITH EXCITATION WAVELENGTHS FOR SiC
NRS
Jia Shao1, Jun Zhao1, Yan Yan1, Shou-Shan Fan2 and Shu-Lin Zhang1
1
2
School of Physics, Peking University, Beijing 100871, China
Department of Physics, Tsinghua University, 100084, China. Email: [email protected]
Keywords: Raman Spectroscopy, SiC Nano-rods
Abstract: The first-order Raman spectrum of SiC NRs reflects its phonon density of state. From
this point we can find the decay of diameter-selective effect. Here we use different incident
wavelengths to study the relation between the frequency variations of LO and TO modes in SiC
NRs and incident photon energy.
SiC materials are used in wide fields, e.g., high-power electronics and photovoltaic and
optoelectronic devices etc. [1, 2]. The phonon behavior is crucial for understanding the behaviors of
the translation etc. in these materials. In our former works [3, 4] we have reported that the nature
and the effect of excitation wavelength on intensity of Raman spectra of SiC nano rods (NRs). Here
we will report on the effect of excitation wavelength on Raman frequencies for SiC NRs.
The samples used are the same to those reported in reference 3. From the TEM images of SiC
NRs we can estimate that the average diameter of these NRs is about 10nm and the length is about
10µm. The X-ray diffraction results show that the sample possesses crystalline structure. The
Raman experiments were performed in Renishaw 1000 Raman microprobe and a Fourier Raman
spectrophotometer (RFS 100/s Bruker NIR-FT spectrophotometer). The sample was excited by
different wavelengths in the back-scattering geometry at room temperature.
In te n s ity
3 C -S iC 5 1 4 n m
1 .4 x1 0
5
1 .2 x1 0
5
1 .0 x1 0
5
8 .0 x1 0
4
6 .0 x1 0
4
4 .0 x1 0
4
2 .0 x1 0
4
0 .0
-2 .0 x1 0
4
650
700
750
800
850
-1
R a m a n S hift(c m )
900
950
1000
1050
1100
Fig 1: SiC NRs Raman spectrum excited at 514nm laser
Fig 1 shows the observed SiC NRs Raman spectrum excited by 514nm line and the
corresponding fitting spectra. From Fig 1 we can see that there are four peaks at 756.8cm-1, 797.0
cm-1, 862.0 cm-1 and 932.7 cm-1. The peak located at 756.8cm-1 is the second order Raman mode of
SiO2, which is the by-product of SiC NRs [4]. The peaks at 797.0cm-1 , 862.0cm-1 and 932.7cm-1
are the TO, interface (IF) and LO modes, respectively.
194
SiC NRs Raman spectra at different excitation wavelength are shown in Figure 2. The
frequency variation of TO and LO with excitation wavelength is shown in Figure 3. We can see
that neither the TO mode nor the LO mode frequency changes with wavelength essentially. This
feature is contrary to that reported in carbon nanotubes (CNTs) [5] and Si nanowires (NWs) [6], in
which the Raman frequency changes with excitation wavelengths.
3C SiC NRs
1100
LO Mode
TO Mode
900
-1
Raman Shift(cm )
1000
1064nm
785nm
633nm
800
700
600
514nm
488nm
500
325nm
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Photon Energy(eV)
700
750
800 850 900 950 1000 1050
-1
Raman Shift cm
Fig 2: Raman spectra of SiC NRs excited at
different wavelength.
Fig 3: Frequency variation of TO
and LO modes with incident photon energy
It has been shown that the first-order Raman spectrum of SiC NRs is relative to the phonon
density of states (PDOS) rater than the phonon dispersion relation, the later CNTs and Si NWs are
relative to. Based on above mentioned scattering mechanism of SiC NRs and notice that the PDOS
will not change with the sample size usually, the observed behavior of Raman frequency with the
excitation wavelength in SiC NRs should be expected even though the sample is nano-scale
material.
Acknowledgements:
We acknowledge the technical assistance from the Nano-Chemical Group of Peking University and
the support from the National Science Foundation of China under grants60390073, 50334040,
50272017, the State Key Lab for Inferred Physics and the Beijing Key Laboratory for
Nano-Photonics and Nano-Structure.
References:
1. B. Delley and E. F. Steigmeier, Phys. Rev. B 47, 1397 (1993).
2. L. Tsykeskov, J. V. Vandyshev, and P. M. Fauchet, Phys. Rev. B 49, 7821 (1994).
3. Shu-Lin Zhang, Bang-Fen Zhu, Fuming Huang, Yan Yan, Er-yi Shang, Shoushan Fan and,
Weigiang Han, Solid State Communications 111, 647–651 (1999).
4. Shu-Lin Zhang, Wei Ding, Yan Yan, Jiang Qu, Bibo Li, Le-yu Li, Kwok To Yue and Dapeng
Yu, APL81(2002):23
5. A. M. Rao, E. Richter, S. Bandow, B. Chase, P. C. Eklund, K. A.Williams, S. Fang, K. R.
Subbaswamy, M. Menon, A. Thess, R. E. Smalley, G. Dresselhaus, and M. S. Dresselhaus, Science,
275, 187 (1997).
6. Bibo Li, Dapeng Yu and Shu-Lin Zhang, Raman spectral study of silicon nanowires. Phys. Rev.
B 59, 1645 (1999).
195