Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học
Trang 1Synthesis of large-scale SiC–SiO 2 nanowires decorated with amorphous carbon nanoparticles and Raman and PL properties
Ryongjin Kima,b, Weiping Qina,*, Guodong Weia, Guofeng Wanga, Lili Wanga, Daisheng Zhnga,
Kezhi Zhenga, Ning Liua
a
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Weiping Qin, 2699 Qianjin Street, Changchun 130012, PR China
b
Semiconductor Research Laboratory, Department of Physics, Kim Il Sung University, Democratic People’s Republic of Korea
a r t i c l e i n f o
Article history:
Received 19 March 2009
In final form 8 May 2009
Available online 13 May 2009
a b s t r a c t
Large-scale SiC–SiO2core–shell nanowires decorated with carbon nanoparticles have been synthesized
on Si substrate using thermal decomposition of ethanol The amorphous carbon nanoparticles on SiO2
shell are of hemispherical configuration hiving a mean diameter of about 20 nm The average size of car-bon nanoparticles estimated from Raman measurement is 23.4 nm close to that evaluated from transmis-sion electron microscopic observation Strong enhancement of blue emistransmis-sion band and appearance of a new yellow emission band were observed in SiC–SiO2core–shell nanowires decorated with carbon nano-particles and their origins were discussed A possible synthesis mechanism of SiC–SiO2core–shell nano-wires decorated with carbon nanoparticles was proposed
Ó 2009 Published by Elsevier B.V
1 Introduction
In connection with the discovery of carbon nanotubes and its
unique properties different from bulk materials in 1991[1], several
kinds of one-dimensional (1D) nanostructures have been
synthe-sized with various methods [1–11] Once various nanowires are
successfully synthesized, the next important step is to control their
architecture, size, and pattern for the practical needs of
nanoelec-tronics and/or nanophotonics[12] This is a crucial step towards
the realization of functional alternative method for micro- and
nanoelectronics and nanophotonics development [13] The SiC–
SiO2 coaxial nanocables were first synthesized by reactive laser
ablation in 1998 and have great potential for applications because
they have the 1D features of both nanocables and nanotubes in the
axial direction and build an ideal semiconductor-insulator
hetero-junction in the radial direction[14] One of the promising
applica-tions of SiC–SiO2nanowires is related to photoluminescence (PL)
property Up to now, numerous publications have been made
assigning to the outstanding ultraviolet-blue emission property
of SiC–SiO2nanowires In previous works, the PL property of SiC–
SiO2nanowires has been studied as-grown[15–19]and no effort
has been paid to control it In present work, we present a novel
SiC–SiO2nanostructure decorated with amorphous carbon
nano-particles and its unique PL property The SiC–SiO2nanowires
dec-orated with carbon nanoparticles have been synthesized on Si
substrate under atmospheric environment Thermal
decomposi-tion of ethanol was used to produce carbon source for synthesis
of this nanostructure Strong enhancement of blue emission band and appearance of a new yellow emission band were observed from this novel nanostructure Structure and Raman property of this nanostructure were characterized and the origins of several
PL bands were discussed A synthesis process of SiC–SiO2 core– shell nanowires decorated with carbon nanoparticles was explained
2 Experiment procedure The synthesis of SiC–SiO2 nanowires decorated with carbon nanoparticles was carried out in a conventional horizontal quartz tube In our synthesis, the Ni(NO3)2-coated Si substrate was di-rectly used as a silicon source 0.09Xcm n-type (1 1 1) Si wafer was firstly etched in 5% HF solution for 2 min to remove the native oxide and then ultrasonically cleaned in acetone bath for 20 min Ni(NO3)2/ethanol solution (0.05 M) was dropped onto Si wafers and dried at 100 °C in the air The catalyst-dropped Si substrates were loaded on a planar alumina crucible and put into the center
of quartz tube Before heating, the furnace was purged by
1000 ml/min Ar gas flow (99.99%) for 1 h After finished purging, the Ar gas flow rate was adjusted down to 300 ml/min and then gaseous ethanol was introduced into the furnace through a bubbler using Ar as carrier gas The flow rate of Ar through the bubbler was held at 10 ml/min The synthesis of SiC–SiO2nanowires decorated with carbon nanoparticles was carried out at 1100 °C for 3 h under the atmospheric pressure After finished synthesis, the surface of the sample synthesized at 300 ml/min Ar gas flow rate (condi-tion-1) was covered with wheat-colored product For comparison, same synthesis route was carried out under 100 ml/min Ar gas 0009-2614/$ - see front matter Ó 2009 Published by Elsevier B.V.
* Corresponding author Fax: +86 431 85168240 8325.
E-mail address: wpqin@jlu.edu.cn (W Qin).
Contents lists available atScienceDirect Chemical Physics Letters
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c p l e t t
Trang 2flow rate The sample synthesized at 100 ml/min flow rate
(condi-tion-2) was covered with white-colored product The products
were characterized by scanning electron microscopy (SEM), X-ray
diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX),
transmission electron microscopy (TEM), high-resolution
trans-mission electron microscopy (HRTEM), selected area electron
dif-fraction (SAED), and Raman spectroscopy Photoluminescence
experiment was carried out at room temperature by using the
He–Cd laser as an exciting source
3 Results and discussion
Fig 1is the XRD pattern of the SiC–SiO2nanowires decorated
with carbon nanoparticles In XRD pattern, five peaks indexed with
(1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) are consistent with the
standard face-centered cubic (fcc) cell of 3C–SiC (JCPDS card No
73-1665), which has the lattice constant of a = 0.4349 nm The
strong SiC (1 1 1) peak implies the predominant growth along
[1 1 1) direction of nanowires A peak centered at 28.6° is due to
Si substrate Two peaks at around 45° are assigned to Ni–Si alloy
[20] A broad peak centered at about 21° is similar to that of pure
silica glasses and attributed to amorphous SiO2.Fig 2shows the
morphologies of two samples obtained at two Ar gas flow rates
It is explicit that large-scale randomly oriented nanowires have
been synthesized on Si substrates in two cases (Fig 2a and b)
High-magnified SEM observation displayed rough surface
mor-phology of the individual nanowire synthesized under the
condi-tion-1 (Fig 2c), while smooth and clean surface morphology was
observed from that synthesized under the condition-2 (Fig 2d)
Further characterization was carried out using TEM From TEM
images, the core–shell nanostructures of two kinds of nanowires
are clearly recognized The SAED patterns recorded from two kinds
of nanowires demonstrate that there are a set of sharp single
crys-tal diffraction spots and an amorphous diffraction ring (two
up-right inserts ofFig 3a and b) Based on HRTEM observation, it is
confirmed that the core is crystalline material and the shell is
amorphous one (an up-left insert ofFig 3a) The distance between
two fringes in core part, is about 0.26 nm close to the (1 1 1)
spac-ing of 3C–SiC (an up-left insert ofFig 3b) Compared to the result
of XRD measurements, the amorphous shell is SiO2 Therefore it is
concluded that our nanowires consist of SiC–SiO2core–shell
nano-structure Like the above result of SEM observation, smooth surface
was observed from TEM image of the nanowires synthesized under
the condition-2 Meanwile, a lot of hemispheric depositions were
found on the surface of the nanowires synthesized under the
con-dition-1 The mean diameter of hemispheric depositions was
eval-uated to be about 20 nm and their concentration on the surface of
SiO2shell was about 120lm2 The interface between hemispheric deposition and SiO2shell layer is clearly observed from HRTEM im-age (an up-left insert ofFig 3a) The distance between two fringes
in HRTEM image of a hemispherical deposition was estimated to be 0.346 nm, which is slightly superior to the theoretical value of d002
interplanar distance in graphite (0.338 nm, JCPDS Card No 00-001-0640) The graphitic plane was found to be roughly oriented along the axial direction of nanowires The corresponding EDX spectra confirm that the carbon content in the nanowires having hemi-spherical depositions on it is higher than that in the nanowires having smooth surface morphology It is obvious that such high carbon content is due to the existence of carbon nanoparticles on the surface of nanowires The existence of carbon nanoparticles
on the surface of nanowires is also identified by Raman measure-ment as shown inFig 4(spectrum (a)) For comparison, the Raman spectrum of the nanowires having smooth surface morphology was presented together (spectrum (b)) In Raman spectra, the peaks having their maximum positions at 793 and 796 cm1are due to SiC TO mode [21–23] The SiC LO mode, which is expected at
972 cm1, is not very clear in two spectra The degradation of the corresponding SiC LO mode is indicative of growth along the [1 1 1] direction by the select rule for zincblende structure polar crystal[24] This result is well agreement with the results of XRD and TEM Two peaks centered at 1337 and 1592 cm1 often called as D- and G-band, respectively confirm the existence of car-bon[22–25] The D-band is known as to appear due to the breath-ing mode of A1g symmetry, which only becomes active in the presence of disorder arising from the crystal boundaries of poly-crystalline graphite [25] The G-band ensues from the stretching mode of ‘sp2’ ðC2
sp C2spÞ bonds (E2gsymmetry in-plane stretching mode of single crystal graphite) [22,25] The D-band grows in intensity with increasing disorder or decreasing crystal size and the ratio of its intensity to that of G-band, ID/IG, is inversely propor-tional to the average size, La, for disordered graphite in the range
2 nm < La< 300 nm [26,27] The intensity ratio of two bands, ID/
IG, can be expressed as follows:
ID
IG
¼CðkÞ
La where C(k) is 4.4 nm for incident laser wavelengths of 514 nm
[28,29] The intensities of the D-band and G-band were determined
by adopting the fitting method used by Ward et al.[25] For spec-trum (a), the average size of graphite structure is estimated to be about 23.4 nm close to the value determined from TEM observation D- and G-band were also appeared in spectrum (b) As can be seen
inFig 4, D- and G-band in spectrum (b) are different from those of spectrum (a) in position, width, and intensity ratio between them For example, the average size for spectrum (b) is estimated to be 2.4 nm This fact implies that structural properties of the carbon
in two kinds of nanowires are different It is well known that an interlayer having high carbon concentration exists at the interface between SiC and SiO2 and this interlayer induce carbon-related peaks (D- and G-band) in Raman scattering[30] It is thought that the carbon-related peaks in spectrum (b) were caused by high car-bon concentration at the interface between SiC core and SiO2shell
Fig 5shows several PL spectra recorded from different nano-wires InFig 5, spectrum (a) and (b) were recorded from two kinds
of nanowires Strong enhancement of blue emission band at about
482 nm (2.58 eV) and appearance of a new yellow emission band centered at about 568 nm (2.17 eV) was observed from the nano-wires decorated with carbon nanoparticles For the nanonano-wires hav-ing smooth surface morphology, the blue emission centered at
482 nm remains as a shoulder and yellow band is absent It is known that the PL from SiC–SiO2core–shell nanowires is mainly originated from their SiO2shells[31] Meanwhile, there are many reports on the blue emission from SiO [32–35] In these reports, Fig 1 XRD pattern of SiC-SiO nanowires decorated with carbon nanoparticles.
Trang 3it is commonly accepted that blue emission from SiO2is mainly
originated from the oxygen deficiency In the SiC–SiO2nanowires
decorated with carbon nanoparticles, it is possible that the higher
defect density is caused in SiO shell by the diffusion and reactions
of carbon species in SiO2matrix[24] The higher defect density will result in the enhancement of blue emission The yellow band can also be regarded in connection with carbon The SiC–SiO2 core– shell nanostructure exists commonly in two kinds of nanowires, Fig 2 SEM images and corresponding EDX spectra of nanowires synthesized at different Ar flow rates.
Fig 3 TEM images of nanowires synthesized at different Ar flow rates.
Trang 4but the nanowires having smooth surface morphology do not
re-veal yellow emission band Therefore, it is reasonable to regard
that the yellow band is originated from SiO2–C system Seo et al
[36]found intense luminescence of carbon-doped silicon-rich
sili-con oxide, which could be tuned from 1.8 to 2.5 eV More recently,
Ishikawa et al.[24]reported yellow–white light emission from
car-bon-incorporated silicon oxide They proposed that electronic
states at the interface between carbon clusters and silicon oxide
and/or carbon-related defects in the SiO2 matrix (for example,
the substitution of oxygen atoms by carbon atoms) could be the
yellow–white emitting sites We believe that the yellow band for
SiC–SiO2nanowires decorated with carbon nanoparticles has same
origin with that reported by Ishikawa et al
At the end, we briefly discuss the synthesis process of SiC–SiO2
nanowires decorated with carbon nanoparticles Ethanol is
ther-mally decomposed into several species, such as CO, CO2, CH4,
C2H4, C2H2, H2O and H2 at the temperature higher than 700 °C
[37] As the temperature goes above 1000 °C, the hydrocarbon
spe-cies are decomposed into H and C[38] Among the several species
produced by ethanol pyrolysis, H2and CO are main gaseous prod-ucts [37] In the catalyst-assisted synthesis, SiC–SiO2 core–shell nanowires grow via a well known vapor–liquid–solid (VLS) mech-anism[26] If the flow rate of Ar dilution gas is slow, carbon is oxi-dized by H2O and O2(contained in Ar dilution gas) or deposit on the inner wall of quartz tube before reaches at the surface of Si substrate As a result, CO becomes only carbon source for growth
of SiC–SiO2core–shell nanowires At high Ar flow rate, some car-bon atoms reach at the surface of Si substrate unoxidized The unoxidized carbon atoms will be deposited on the surface of SiO2 shell and thus, nanoparticles will be formed on SiO2shell Further increasing of Ar flow rate result in the large amount of carbon depositions on the surface of SiO2shell and these nanowires did not emit measurable PL
4 Conclusion SiC–SiO2core–shell nanowires decorated with carbon nanopar-ticles have been synthesized using thermal decomposition of eth-anol The existence of carbon nanoparticles induced strong carbon-related bands in Raman spectrum The enhancement of blue emission band and a new yellow band were observed from SiC–SiO2core–shell nanowires decorated with carbon nanoparti-cles and considered to relate to the enhancement of oxygen defi-ciency and appearance of carbon-related default level due to carbon diffusion into SiO2and electronic state at the interface be-tween carbon nanoparticles and SiO2shell
Acknowledgement This research was supported by Natural Science Foundation of China (Grant Nos 50672030 and 10874058)
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